The smallest such prime...

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  • Опубліковано 2 жов 2024
  • We look at a nice number theory problem.
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КОМЕНТАРІ • 173

  • @DavidCorneth
    @DavidCorneth 3 роки тому +109

    4:58 "See what I did there?" mp = Michael Penn?

    • @goodplacetostop2973
      @goodplacetostop2973 3 роки тому +16

      🅱️ = Michael Penn

    • @sababugs1125
      @sababugs1125 3 роки тому +2

      I thought it meant math professor

    • @nelsblair2667
      @nelsblair2667 3 роки тому +2

      I always teach my students to use their names in the selection of their variables. I struggled to guess why this gymnast 🤸‍♂️ kept using m. It all makes sense to me now.

    • @srijanbhowmick9570
      @srijanbhowmick9570 3 роки тому +3

      @@nelsblair2667 LMAO Gymnast 😂

  • @laszloliptak611
    @laszloliptak611 3 роки тому +89

    For subcase 1 you didn't need to involve x at all because (p-1)^2

    • @niceroundtv
      @niceroundtv 3 роки тому +9

      That's actually quite elegant.

    • @megauser8512
      @megauser8512 3 роки тому +4

      Yeah, that makes sense.

  • @gianlucamerlino
    @gianlucamerlino 3 роки тому +138

    If anyone is wondering: a=6083, b=78, p=23

    • @tharagleb
      @tharagleb 3 роки тому +20

      I wish every math YTer would go back and show that their answer actually satisfies the initial problem, at least where applicable.

    • @KiLLJoYYouTube
      @KiLLJoYYouTube 3 роки тому +2

      how did you find that out?

    • @wcvp
      @wcvp 3 роки тому +5

      @@KiLLJoYUA-cam I was curious if it would take me long to write a shitty way of finding it in C, so I found it that way.

    • @megauser8512
      @megauser8512 3 роки тому +1

      Nice!

    • @SlipperyTeeth
      @SlipperyTeeth 3 роки тому +2

      Also, a=b^2-1

  • @tomatrix7525
    @tomatrix7525 3 роки тому +22

    Your presentation style is unbeatable. So enthralling, It’s usually you find a good solution but bad presentation, or the inverse, but rarely both.

  • @Dilip077
    @Dilip077 3 роки тому +57

    National mathematics Day :)

    • @Uni-Coder
      @Uni-Coder 3 роки тому

      Can mathematics be national?

    • @Abhi-ib4fr
      @Abhi-ib4fr 3 роки тому

      @@Uni-Coder Yes it is in India

  • @SONUKUMAR-mb2sp
    @SONUKUMAR-mb2sp 3 роки тому +44

    National mathematics day. 22 dec.🇮🇳🇮🇳🇮🇳

  • @swastikadeb8605
    @swastikadeb8605 3 роки тому +20

    Happy national mathematics day! :)
    Nice problem!

  • @nullplan01
    @nullplan01 3 роки тому +18

    That was a fun one, but the hint totally gave the game away. :-) So the problem statement is equivalent to p³ = b⁴ - a² = (b² - a)(b² + a). Since p is prime, p³ can only be split up two ways, p * p² and 1 * p³. And since a and b are positive integers, b² - a < b² + a, so the factors can be mapped to the terms in only two ways.
    Case 1: b² - a = p, b² + a = p²
    Add up both equations yields 2b² = p(1+p). The case p=2 can be discarded quickly, so p must be odd. Therefore 1+p is even, so we can divide the equation by 2 and get
    b² = p (1+p)/2 (where the last term is an integer). Therefore b² is divisible by p. Since p is an integer, it follows that b is divisible by p. So there is some integer n with b = np, so
    n² p² = p (1+p)/2
    n² p = (1+p)/2.
    So (1+p)/2 is divisible by p. But (1+p)/2 < p for all p >= 3, so that can't be right. So there can be no solutions in this case.
    Case 2: b² - a = 1, b² + a = p³
    Add up both equations yields 2b² = 1 + p³ --> b² = (1 + p³)/2. This again discounts p=2 as possibility, since in that case the RHS is not natural. I had no idea how to proceed from here, so I just wrote down a table of prime numbers, their cubes, the RHS of the equation, and their square roots. By checking every known prime number, I was able to find that p=23 leads to b=78, and a=6083. A quick check later confirmed this to be a solution. And since the other case found none, and this case found none before this one, the smallest solution is:
    6083² + 23³ = 78⁴

    • @梁偉康-d9k
      @梁偉康-d9k 2 роки тому

      Hvjvb

    • @梁偉康-d9k
      @梁偉康-d9k 2 роки тому

      G6f

    • @RexxSchneider
      @RexxSchneider Місяць тому

      I did it this way as well, but before I watched the video, so I didn't have the hint to spoil it, although these sort of problems almost always involve unique factorisations of expressions.
      Fortunately, I tried the b² - a = 1, b² + a = p³ case first and used an Excel spreadsheet to calculate b = SQRT((1+p^3)/2) against a column of primes, showing p=23 and b=78 as a solution. For the second case, I was then able to stop calculating p=(SQRT(8*b^2+1)+1)/2 once p grew larger than 23. I often "cheat" and use Excel for trial-and-error solutions as it's trivial to "knock up" a quick series of calculations, especially if you keep a text file with a column of primes to paste as your starting point.

  • @JSSTyger
    @JSSTyger 3 роки тому +5

    14:30 *Michael Penn:* Tony, there was no other way. We're in the endgame now...

  • @goodplacetostop2973
    @goodplacetostop2973 3 роки тому +43

    16:42 My comment was shadowbanned by UA-cam so let’s try this again...
    Geometry homework today! Good luck and have a good day...
    Squares are constructed externally on the sides of an arbitrary quadrilateral. Show that the line segments joining the centers of opposite squares lie on perpendicular lines and are of equal length.

    • @goodplacetostop2973
      @goodplacetostop2973 3 роки тому +7

      Oh I see, it didn’t like the link. So the solution is at this adress : qbyte.org/puzzles/p062s.html

    • @tonyhaddad1394
      @tonyhaddad1394 3 роки тому +1

      Thank u

    • @blazedinfernape886
      @blazedinfernape886 3 роки тому

      Ahhh midpoint theorem. Brings me back to middle school days!

    • @goodplacetostop2973
      @goodplacetostop2973 3 роки тому +3

      @@tonyhaddad1394 You're welcome

    • @goodplacetostop2973
      @goodplacetostop2973 3 роки тому +2

      @@blazedinfernape886 Also known as van Aubel's Theorem: mathworld.wolfram.com/vanAubelsTheorem.html

  • @koenth2359
    @koenth2359 3 роки тому

    In subcase 1, we need not consider x at all to see that y^2=p^2-p+1 implies (p-1)^2 < y^2 < p^2

  • @neilgerace355
    @neilgerace355 3 роки тому +27

    p = 2 doesn't work for case 2 because if b^2 + a = 8, and b^2 - a = 1, then 2a = 7. But a is an integer: contradiction.

    • @davidblauyoutube
      @davidblauyoutube 3 роки тому +15

      Yes, it was premature to carry over this "fact" from the first case. It had to be proved separately for the second case.

    • @nathanisbored
      @nathanisbored 3 роки тому +10

      @@davidblauyoutube he technically assumed it outside of both cases, left as an exercise to the viewer from the given equation at the start. tho he didnt mention it until it became relevant in case 1

    • @megauser8512
      @megauser8512 3 роки тому +2

      If you just take those 2 equations as they are, and then eliminate a, then you get 2*b^2 = 9, which is a contradiction, since we are given that b is an integer.

  • @thomasoa
    @thomasoa 3 роки тому +7

    The relatively prime case is easier. You get (p-1)^2

  • @hsjkdsgd
    @hsjkdsgd 3 роки тому +9

    Thanks to the guy who invented internet. I get to watch and learn such cool stuff.

  • @vinc17fr
    @vinc17fr 3 роки тому +8

    Once one has p³ = 2b² − 1, a quick analysis modulo 8 yields p ≡ ±1 mod 8. Then one just needs to test p = 7, 17, then 23 works. That's much faster, but Michael's solution could have been faster if the minimum p were much larger.

    • @happygimp0
      @happygimp0 3 роки тому

      Writing a short program and test primes till it finds a valid answer would be much faster.

    • @vinc17fr
      @vinc17fr 3 роки тому +2

      @@happygimp0 One doesn't always have a computer or even a calculator, in particular for math Olympiads.

  • @themathhatter5290
    @themathhatter5290 3 роки тому +4

    So I made a spreadsheet of the possibilities as we found in the only subcase that works.
    x, p=6x^2-1, y=sqrt((p^2-p+1)/3)
    As it turns out, x=2 is the only x

  • @arekkrolak6320
    @arekkrolak6320 Рік тому +1

    You eliminated p=2 only in respect to the first factorization, cannot bring that result into another case I think

  • @Risu0chan
    @Risu0chan 2 роки тому +1

    We've got the smallest solution, but is it the only one? I found no other below 10^15 (Python is fast!). I suspect it's the only solution.

  • @themathhatter5290
    @themathhatter5290 3 роки тому +10

    Sounds like you chopped off some explanation at the beginning there!

  • @zygoloid
    @zygoloid 2 роки тому +1

    Looks like my approach was basically the same as Michael's:
    a²+p³=b⁴
    p³=(b²+a)(b²-a)
    Case 1:
    b²+a=p²
    b²-a=p
    2b²=p(p+1) p would make RHS ≥ (p+1)p, so n+1=p, but then RHS = p(p-2)).
    Case B: gcd = 3:
    p+1=6m²
    p=6m²-1
    Try values of m:
    p=5: no
    p=23: yes, 23²-23+1=507=3.13²
    p=23, b=78, a=6083

  • @MizardXYT
    @MizardXYT 3 роки тому +5

    p=23 is the only prime that works for p < 1.000.000.

    • @IanXMiller
      @IanXMiller 3 роки тому +1

      Yeh I noticed that too. Wonder how to prove its a unique answer.

    • @davidbrisbane7206
      @davidbrisbane7206 3 роки тому

      Interesting.

  • @iamadooddood4331
    @iamadooddood4331 3 роки тому +2

    Case 1 can be made easier by: since p | b, p² | b², but p² ∤ p(p+1), so we have a contradiction.
    Also, I thought of finding all values of p that satisfy this equation and there may actually not be so many solutions. I tried to narrow down the possibilities of p and got this:
    p + 1 = 6x²
    p = 6x² - 1
    3y² = p² - p + 1 = (36x⁴ - 12x² + 1) - (6x² - 1) + 1 = 36x⁴ - 18x² + 3
    y^2 = 12x⁴ - 6x² + 1
    Since y² is odd, y ≡ 1 (mod 8), thus x is even.
    Let 2w = x, then p = 24w² - 1 (this means that p ≡ 23 (mod 24))
    y^2 = 192w⁴ - 24w² + 1 (this polynomial cannot be factorised without going into complex numbers)
    Additionally, since for any perfect square k^2 ≡ -1, 0, 1 (mod 5), w ≡ -1, 0, 1 (mod 5). (And in particular, y ≡ 1 (mod 100) if w ≡ 0 (mod 5) and y ≡ 69 (mod 100) if w ≡ ±1 (mod 5), though I'm not sure how helpful this is.) In fact, I plugged in values for w = 4, 5, 6, 10, 11 (9 gives a composite value for 24w² - 1) and none of them gave a perfect square for 192w⁴ - 24w² + 1.

  • @davidepierrat9072
    @davidepierrat9072 3 роки тому +9

    13:29 I find it sort of confusing bounding y^2 in terms of x. Could have just written as (p-1)^2

  • @asklar
    @asklar 3 роки тому +4

    I feel no shame in admitting I wasn't able to solve this on my own.
    Ok maybe a little.

    • @thatkindcoder7510
      @thatkindcoder7510 3 роки тому

      But guess what I could solve it
      (After seeing the solution)

  • @VIRUS200086
    @VIRUS200086 3 роки тому +2

    We proved that IF the solution exists, thus it has to be 23. But we could have that the solution doesn't exist, right?

  • @emanuellandeholm5657
    @emanuellandeholm5657 3 роки тому

    My first thought is p^3 = b^4 - a^2 = (b^2)^2 - a^2 = (b^2 - a)(b^2 + a). Examine small primes in order to find a solution. (b^2 -a) is obviously less than (b^2 + a) so it has to be either 1 or p. If we assume p is 2, we get b^2 - a = 1 or 2 and b^2 + a = 2 or 4. This gives 2 b^2 = 3 or 6, which has no solutions in Z. So p must be an odd prime. Just check all of the small ones until you get a hit.
    p|b^2 => p|b, this is only true for prime p. Obviously 50|100 but 50 does not | 10.

  • @johnloony68
    @johnloony68 3 роки тому +3

    37,002,889+12,167=37,015,056 or 6,083^2+23^3=78^4

  • @dionisis1917
    @dionisis1917 3 роки тому +2

    Find all m,n natural number such that
    (3^m) +23=2^n

  • @samyakmahapatra9154
    @samyakmahapatra9154 3 роки тому +2

    This one required some patience, but nonetheless nice problem

  • @martinnyberg8174
    @martinnyberg8174 3 роки тому +6

    Is the symbol Michael uses for "contradiction" a standard one? I've never seen it before. 🤔

    • @SuperMerlin100
      @SuperMerlin100 3 роки тому +1

      No it isn't. The "standard" symbol is an upsidedown "T".

    • @martinnyberg8174
      @martinnyberg8174 3 роки тому +1

      @@SuperMerlin100 I've seen two implication arrows pointing against each other used for "contradiction". Are there more in use that I've missed?🤔☺️

    • @djvalentedochp
      @djvalentedochp 3 роки тому +1

      michael does it like this -->

    • @martinnyberg8174
      @martinnyberg8174 3 роки тому +1

      @@djvalentedochp Really? Looks like a + with a strike-through to me. 🤔

    • @nelsblair2667
      @nelsblair2667 3 роки тому

      That doesn’t add up (plus sign with strike through it, joke)

  • @philippenachtergal6077
    @philippenachtergal6077 3 роки тому

    ok so (I'm omitting all the exponentiation symbols : p3 means p^3 etc.. but 2p still means 2*p.)
    p3 = b4 - a2 = ( b2-a)(b2+a)
    So either:
    1) b2-a=1 and b2+a=p3
    or 2) b2-a = p and b2+a = p2
    Since b2+a == b2-a + 2a -->
    For 1) we get that 1+2a = p3 so a = ( p3-1)/2 and b2 = (p3+1)/2
    For 2) we get p + 2a = p2 --> a = (p2 - p) /2 = p (p-1) / 2 and b2 = p (p+1) / 2
    Now, by successively trying for p=2,3,... ; I do get that the smallest solution for p is p=23, a=6083, b=78
    I can get to that solution with just pen and paper but it is tedious. I used a spreadsheet and got the solution in minutes. Using a simple calculator would also get me the solution in just a few more minutes.
    I don't really know how to reduce the amount of manual calcul to do to get to the solution.

  • @Qermaq
    @Qermaq 3 роки тому +3

    So 6083^2 + 23^3 = 78^4. Yay science!

  • @The1RandomFool
    @The1RandomFool 2 роки тому

    I ran a Python script to check all possible combinations of a, b, and p up to a certain b-value, and I only found one combination: p=23, a=6083, and b=78 when searching up to a b-value of 3000. The numbers are getting quite large and the script runs so slow at that point, even with all 16 threads running on my Ryzen 5800X.

  • @nakamakai5553
    @nakamakai5553 3 роки тому +7

    He always knows a good place to stop

  • @MrGeorge1896
    @MrGeorge1896 3 роки тому

    When I came to case 2 I just started to calc sqrt((p^3+1)/2) for small primes which gave me 23 as the smallest solution. I had more test cases but got the solution in less time.
    btw: I as far as I can see 23 is also the ONLY prime number which solves this problem.

    • @jeanf6295
      @jeanf6295 2 роки тому +2

      No solution besides 23 bellow 100 000 000 using python

  • @jasonwoolf
    @jasonwoolf 3 роки тому +2

    I was delighted to notice that for Case 1, we find that b^2 equals p(p+1)/2, which is 1+2+...+p (a.k.a. the triangle number of p). This harkens back to the balancing number video. It's easy to show that balancing numbers are the square roots of triangle numbers (though this wasn't mentioned in the video). Thus for Case 1, b has to be a balancing number! Now, it appears that the sequence of numbers whose triangle number is a perfect square (8, 49, 288, 1681, 9800, 57121, ...) contains only even numbers alternating with odd perfect squares -- i.e. no primes! Thus, Case 1 yields no solutions. For my next trick, I need to prove that last assertion...

    • @jasonwoolf
      @jasonwoolf 3 роки тому

      This has led me to a surprising but easily provable observation: The difference of the squares of consecutive triangular numbers is the cube of the triangular root of the larger. In other words:
      (1+...+n)^2 - (1+...+(n-1))^2 = n^3.
      I suppose this is already well known to real number theorists. :-)

    • @themathhatter5290
      @themathhatter5290 2 роки тому

      @@jasonwoolf It has been a year, but perhaps you want to look at Michael's sweatshirt.

    • @jasonwoolf
      @jasonwoolf 2 роки тому

      @@themathhatter5290 Ha, yes, I’ve noticed it in many videos over the past year, but I missed it while watching and commenting on this one. I was an MP newbie back then.

  • @suparnoroy118
    @suparnoroy118 3 роки тому +3

    nicely donee !! :) great job !

  • @luisisaurio
    @luisisaurio Рік тому

    I need help
    Suppose p>2 and a and b are coprime with p then
    Rearrenging we get that p^3=(b^2)^2-a^2
    By LTE we get that 3=v_p((b^2)^2-a^2)=v_p(b^2-a)+v_p(2). Since v_p(2) for p>2 is 0 then v_p(b^2-a)=3. In other words p^3|b^2-a and that b^2-a >= p^3
    Since p^3=(b^2-a)(b^2+a)>=p^3(b^2+a) which means that b^2+a=1 but this is ridiculous.
    What did I do wrong?

  • @piman9280
    @piman9280 3 роки тому +1

    Penn has become a Teller - magical problem!

  • @Czeckie
    @Czeckie Рік тому

    any idea about any other possible solutions? Exhaustive search for x up to 10^8 yielded none.

  • @deltalima6703
    @deltalima6703 2 роки тому

    I hate this problem. Relentess hopeless handwaving that makes no sense. Just like the worst problems from school.

  • @anggalol
    @anggalol 3 роки тому +2

    Lol why I think you should put your silver play button above the blackboard

    • @anggalol
      @anggalol 3 роки тому

      @Jon Jukoba yeah lol

    • @vaxjoaberg9452
      @vaxjoaberg9452 3 роки тому

      How is mathematics not entertaining?

  • @leif1075
    @leif1075 3 роки тому

    Wait a minute at 2:25 why can't he switch the cases he forgot..you can have p divide the bigger term and p squared divide the b squared minus a..I dont see why not. Same with p cubed and 1 factors..

    • @evanmurphy4850
      @evanmurphy4850 3 роки тому +2

      Essentially because p^2 > p and b^2+a > b^2-a, they need to match up if that makes sense. The two bigger terms have to be equal, and the two smaller terms have to be equal.

  • @mcwulf25
    @mcwulf25 2 роки тому

    Like it. Especially because the answer isn't 0,1 or 2.

  • @helo3827
    @helo3827 3 роки тому +1

    How do you prove the GCD trick?

  • @helo3827
    @helo3827 3 роки тому +1

    what is the inspiration to look at the GCD of the 2 numbers?

    • @tomasebenlendr6440
      @tomasebenlendr6440 3 роки тому +1

      If you ask why there should be small number of cases of outcome of GCD, then I have no clue.
      But if you accept that you just try to factorize X=p^2-p+1 and Y=(p+1)/2 to further restrict number of possibilities, then from XY being perfect square you know that X=x^2*GCD(X,Y) and Y=y^2*GCD(X,Y) for x and y coprime natural numbers (i.e., x in N, y in N, GCD(x,y)=1). (You can imagine full factorization of X and Y and see how are the primes distributed.)
      But you can as well stop at 2b^2 = p^3+1, rewrite it as b = sqrt( (p^3+1)/2 ) and start testing primes. p=23 is ninth prime, not being too far if you have no other clue how to restrict number of primes to test.

  • @KiLLJoYYouTube
    @KiLLJoYYouTube 3 роки тому +1

    i'm convinced the 23 enigma is real

  • @yahav897
    @yahav897 3 роки тому +1

    Hi everyone, I'm not sure if this is the place, but I've wanted to ask for advice. I like maths, and I really do find them enjoyable, through videos or pop-sci books, and I think I'd be pretty good at it; however, due to personal issues I can't study that (or related topics) in college right now. What are some books, videos, lectures to help me get started learning mathematics?
    I'm not sure if I'm going to major in it, but I really do want to learn more of it!
    P.S. Does anyone know about career options?

    • @jomama3465
      @jomama3465 3 роки тому

      I think you should start with some basic algebra and elementary geometry, that got me hooked on math. And don't forget to practice arithmetic because it'll be useful especially when you're studying algebra. Give it a try, math is fun.

    • @yahav897
      @yahav897 3 роки тому

      @@jomama3465 I finished highschool, and I think my knowledge is about the level of AP Calc AB + algebra 1 (on khan academy), if that helps somehow

    • @adityaekbote8498
      @adityaekbote8498 3 роки тому

      @@yahav897 what are you doing now mate for other people and you the answer maybe to start with linear algebra (good books on linear algebra are linear algebra by insel friedberg and algebra by serge lang or algebra by artin) or you might try differential equations (math sorcerer has video lectures on it I am too lazy to provide a link sed) or you might wanna go for real analysis ( best books are mathematical principles or principles in mathematics I don't remember although it is very famous by the author Walter Rudin or understanding analysis by Stephen Abbott is also a good one ) or try number theory (best book is by david Burton)

  • @riccardomagnolfi7164
    @riccardomagnolfi7164 3 роки тому +2

    I really enjoyed this problem, Michael, thank you!
    I would like to suggest another solution, that doesn't need the use of the gcd's tricks.
    Same procedure up to minute 06:55, so we know 2b²=p³ + 1.
    Since p is odd, let's write p=2n-1. Then we have:
    2b² = p³ + 1 = (8n³ - 12n² + 6n - 1) + 1 = 8n³ - 12n² + 6n
    b² = 4n³ - 6n² + 3n = n × (4n² - 6n + 3)
    Let's factorize n in prime numbers:
    If c is a prime dividing n, then c also divides 4n² - 6n, so c divides (4n² - 6n + 3) ONLY IF c=3.
    Any prime d≠3 dividing n IS NOT a factor of (4n² - 6n + 3), so d must have an even exponent in the factorization of n, else n×(4n² - 6n + 3) CANNOT be a perfect square.
    In other words, the only prime number than can appear with an odd exponent in the factorization of n is the number 3.
    So we must search for n such that 2n-1 is prime and:
    1) n is a perfect square, OR
    2) n is 3 times a perfect square
    Then n is in the set {3, 4, 9, 12, 16, ...}
    The first value for n in this set that works is n=12:
    n = 3×2²
    (4n² - 6n + 3) = 4×144 - 6×12 + 3 = 507 = 3×169 = 3×13²
    b² = (3×2²) × (3×13²) = (2 × 3 × 13)²
    That yields p = 2×12 - 1 = 23

  • @benheideveld4617
    @benheideveld4617 3 роки тому +1

    p=23, OK, but what about a and b?

  • @mayonakao2488
    @mayonakao2488 2 роки тому

    Jesus, I did this thinking before the video, and thought I had to prove each prime. Glad to know I’m not the only one totally lost on that front. Also, this is my solution using the last digits of numbers as my contradictions.
    The first step is the factoring then proving that bb-a = 1 not p
    Since that would say p(p+1)=2bb which means p2M=2bb or pM=bb so p=b since m doesnt have p as factor since ita smaller than p, but p=b means p+1 = 2p
    Ppp= bb + a And bb - a = 1
    So ( ppp+1 )/2 = bb, a square
    A square only ends in 014569
    But primes only end in 1379
    Then, this function shows that the only primes that can work are those ending in 13 or 9, and bb ends in 014
    Ppp -1 )/2 = a
    But bb - a = 1
    Therefore a can only end in 903
    The only primes that do this end in 0 or 3
    This was tricky: if a ends in 0
    Then p ends in 1
    But then appp = ???10 = 2aa +a = ??00 + a
    Therefore a ends in 10
    But if a ends in 10, bb ends in 11
    Since bb-a = exactly 1
    But no number squared ends in 11
    Proving that is confusing but generally the first two digits of square are created by {aa+20ab} where a and b are the digit of the ones and the tens places. There is no combination of the digits a and b that make that end in 11, since a must be 1 or 9, but that means 1+20b ends in 11, so 2b ends in 1, or 81+180b ends in 11 which brute force shows simply doesnt.
    Therefore only primes ending in 3 will ever work.
    There is no way to show what the next prime beyond 23 would be, but I tried: p is z10 +3
    And putting that into ( ppp+1) /2 equals a square, perhaps you could show which values of z are possible and more importantly why.

    • @mayonakao2488
      @mayonakao2488 2 роки тому

      Actually, it might be possible to prove that similar formulas have only one solution, and derive from that that this formula also has one solution. Cuz comments say its 1 in an eternity to find the next solution, implying its just 23.

  • @Tiqerboy
    @Tiqerboy 3 роки тому

    The way I discarded the first case where 2b² = p(p+1) you have b² = p(p+1)/2. Geometrically that can't work since a stick of length p that can't be broken down (because p can't be factored) to fit into a square of size b², because square root of b is necessarily smaller than p (geometric mean lies between p and (p+1)/2 ). There you go. Not as elegant as how you gave it, but it's what I came up with.

  • @tonyhaddad1394
    @tonyhaddad1394 3 роки тому +2

    Micheal you are my drug 🤩🤩🤩🤩🤩🤩🤩🤩🤩🤩🤩🤩🤩🤩

  • @pwmiles56
    @pwmiles56 3 роки тому

    Lazy way: taking b^2-a=1, b^2+a=p^3, solve for b^2 = (p^3+1)/2. p=23 is the first prime that works, b=78, a=6083

  • @Tornado.363
    @Tornado.363 2 місяці тому

    please someone explain this 14:17

  • @prithujsarkar2010
    @prithujsarkar2010 3 роки тому

    Prof Michael
    Can you check this problem out
    Define a positive integer $n$ to be a factorial tail if there is some positive integer $m$ such that the decimal representation of $m!$ ends with exactly $n$ zeroes. How many positive integers less than $1992$ are not factorial tails?
    It's such a beautiful question :) also involves floor functions

    • @samyakmahapatra9154
      @samyakmahapatra9154 3 роки тому +1

      🤔🤔 I am able to see the legendary legendre formula here

  • @onderozenc4470
    @onderozenc4470 3 роки тому

    We could also factorize p as p^(1 / 4 ) x p(3 /4) , can't we ?

  • @stkhan1945
    @stkhan1945 Рік тому

    ..amazing ..so many layers of concepts revealed as Michael Penn went on..

  • @user-A168
    @user-A168 3 роки тому

    Good

  • @Cjendjsidj
    @Cjendjsidj 2 роки тому

    4:58 mp michael penn

  • @jamesfortune243
    @jamesfortune243 2 роки тому

    Great problem!

  • @アヤミ
    @アヤミ 3 роки тому

    33333rd view, lol

  • @niom9446
    @niom9446 2 роки тому

    that was amazing

  • @ethanbartiromo2888
    @ethanbartiromo2888 2 роки тому

    MP = Michael Penn

  • @jensbeckmann5349
    @jensbeckmann5349 2 роки тому

    Which semester is this?

  • @bekhaddaderrar2111
    @bekhaddaderrar2111 3 роки тому

    Wow this is something beautiful and incredible thank you for this video

  • @barbietripping
    @barbietripping 3 роки тому +3

    I like to write my thoughts before seeing too many hints:
    p^3=(b^2 - a)(b^2 +a)
    so b^2-a must be 1 and b^2+a must be minimized. I imagine just picking the smallest square other than 1 to be equal to b^2 would work. b=2, a=3 so p=7
    Edit: I see I just forgot the cubed ... so b^2-a does not have to be 1.
    Sorry to everyone who has to read my bad ideas

  • @perappelgren948
    @perappelgren948 3 роки тому

    Great!

  • @adharshsb38
    @adharshsb38 3 роки тому

    Can anyone help mewith the GCD trick....can anyone help..plsssss

    • @kostaspapadopoulos1480
      @kostaspapadopoulos1480 3 роки тому

      Let's say d=gcd((p+1)/2,p^2-p+1) and d'=gcd(p+1,p^2-p+1) for p an odd prime. Then d/(p+1)/2=>d/(p+1)=>d/d'. Because p^2-p+1 is odd, d' has to be odd, thus gcd(d',2)=1. So d'/(p+1)=>d'/2(p+1)/2 and gcd(d',2)=1, that means d'/(p+1)/2=>d'/d. So d=d' because they divide each other.

    • @kostaspapadopoulos1480
      @kostaspapadopoulos1480 3 роки тому

      For the second trick we have that gcd(a,b)=gcd(a,b-ka), because d/a and d/b=>d/b-ka=>d/d' and d'/a=>d'/ka, so d'/ka and d'/b-ka=>d'/ka+b-ka=>d'/b=>d'/d, so again d=d'.

  • @emilyliedtke7059
    @emilyliedtke7059 3 роки тому +2

    Honestly one can just start putting in primes after we get the relatively simple criterion that (p^3+1)/2 has to be a square, since we're just looking for the smallest solution... not as elegant, but way faster

  • @kostaspapadopoulos1480
    @kostaspapadopoulos1480 3 роки тому

    Very nice problem involving lots of elementary number theory!

  • @riadsouissi
    @riadsouissi 3 роки тому +4

    After I got 2*b^2=p^3+1, I just checked b and p mod 3,5,8. And had the following conditions:
    p must be 1,5,7 mod(8)
    p cannot be 1 mod(3)
    p cannot be 2 mod(5)
    the first prime that verifies these 3 conditions is p=23 which also verifies the equation.

    • @reeeeeplease1178
      @reeeeeplease1178 2 роки тому

      How do you get the "p cannot" parts?
      Perfect squares mod 3 are 0 and 1
      So 2b^2 is 0,2 mod 3
      => p = 2b^2 - 1 is -1,1 or 1,2 mod 3
      Perfect squares mod 5 are 0,1,4
      So 2b^2 can be 0, 2, 3 mod 5
      And thus p can be 1, 2, 4 mod 5

    • @riadsouissi
      @riadsouissi 2 роки тому

      @@reeeeeplease1178 for mod 3, it was a typo, p cannot be 0 mod (3), so basically eliminates 3.
      For mod 5, if p is 2 mod 5, then p^3+1 becomes 4 mod 5, which 2b^2 cannot be.
      Still, with only mod 8 and mod 5 conditions, we can easily eliminate primes up to p=23.

    • @reeeeeplease1178
      @reeeeeplease1178 2 роки тому

      @@riadsouissi ohhhh i missed the p^3

  • @samegawa_sharkskin
    @samegawa_sharkskin 3 роки тому

    best video ever! i could actually see myself solving this myself

  • @ΒασιλικηΚοτσαλη
    @ΒασιλικηΚοτσαλη 3 роки тому +2

    Aren't there 2 more cases; b^2-a being p^2, b^2+a being p and b^2-a being p^3, b^2+a being 1?

    • @Victor_Marius
      @Victor_Marius 3 роки тому +1

      No, because -a < a since a > 0, and b^2-a < b^2+a so p^x < p^y and x < y, where (x, y) is (1, 2) or (0, 3)

  • @youknowme3346
    @youknowme3346 2 роки тому

    9:48 isnt gcd of p+1 and 3 = 1?

  • @vbcool83
    @vbcool83 3 роки тому

    One of the best videos I have found!

  • @happygimp0
    @happygimp0 3 роки тому

    Would it not be faster to write a script that tests different primes till it finds a solution?

    • @happygimp0
      @happygimp0 3 роки тому +1

      def getPrimes(max=2000):
      yield 2
      r=3;
      while r

  • @kqp1998gyy
    @kqp1998gyy 3 роки тому

    Beautiful ❤️

  • @pbj4184
    @pbj4184 3 роки тому

    8:30 I think this is the proof of the property used but is it correct?
    Let gcd(a,b)=k and let gcd(a,b+an)=m where n is an integer
    By definition of the gcd, we know k is the greatest natural number such that k|a and k|b simultaneously. Similarly, m is the greatest natural number to divide both a and b+an at the same time.
    But if m|a, then m|b+an implies m|b as a*n is a multiple of m -and therefore m is the greatest† number to divide both a and b and so is their gcd and therefore m=k- QED
    But I think just because m|a and m|b doesn't imply it is the _greatest_ number to do so and it might not be the gcd of a and b necessarily. Can someone please help me?
    *EDIT* : The crossed-out part is the non-sequitur part of the original attempt at a proof
    †We know both m and k divide a and b and k is the greatest possible number that can do so. So m

    • @lucaseikifujii2420
      @lucaseikifujii2420 3 роки тому

      Search for gcd Euclid lemma

    • @wasselkun5015
      @wasselkun5015 3 роки тому +2

      From what you did you can conclude that k|m, so now show that m|k and you are done.

    • @ichtusvis
      @ichtusvis 3 роки тому +2

      An easy way to convince yourself of the symmetry of this argument (i.e. that it gives both m|k and k|m), may be to substitute c=b+na and run through the exact same logic

    • @pbj4184
      @pbj4184 3 роки тому +1

      @@ichtusvis I see, we get b=c-an and so the roles of m and k are reversed here and since we saw one divides the other in the original comment and we get the opposite here and so m|k, k|m and therefore m=k
      Thanks a lot for commenting :)

    • @pbj4184
      @pbj4184 3 роки тому +2

      @@wasselkun5015 Your comment with the comment below yours helped a lot. Thanks a lot for commenting :)

  • @maxblack493
    @maxblack493 3 роки тому

    Good!

  • @nataliarose964
    @nataliarose964 3 роки тому

    Are there infinitely many such primes?

    • @tomasebenlendr6440
      @tomasebenlendr6440 3 роки тому

      Is there any other such prime? If my computer program is correct, then no number 3

  • @leif1075
    @leif1075 3 роки тому

    Why would anyone think to break up p into p squared times p ..i dont see anyone thinking of that..

    • @ahzong3544
      @ahzong3544 3 роки тому

      It's a common trick to think about prime factorisation of a power of prime (p^n) like this.

    • @leif1075
      @leif1075 3 роки тому

      @@ahzong3544 well tricks are dirty and sneaky and shouldn't count..

    • @opjdwaiodhawduoawbiud
      @opjdwaiodhawduoawbiud 3 роки тому

      @@leif1075 there is no dirty way to solve a puzzle, only a solution

    • @ahzong3544
      @ahzong3544 3 роки тому

      @@leif1075 If you mean that tricks shouldn't be counted as a solution, then this statement does not seem to be true.
      To be good at solving math olympiad problems, it is necessary to master certain commonly used tricks. These tricks can then help you do well in math olympiad.
      I won't say tricks are dirty, since it doesn't violate the rules. They are something you can have in your mind to help you when solving math olympiad problems.

  • @tonyhaddad1394
    @tonyhaddad1394 3 роки тому +1

    So don t be late 🙏

  • @watchaccount
    @watchaccount 3 роки тому +3

    that was not a good places to stop. You have tested the values in the equation.

  • @AnatoArchives
    @AnatoArchives 3 роки тому +5

    Bruh why am I wasting my early teenager days with calculus.

    • @bourhinorc1421
      @bourhinorc1421 3 роки тому +2

      *with "arithmetic"

    • @AnatoArchives
      @AnatoArchives 3 роки тому +1

      @@bourhinorc1421 *cries hysterically*

    • @brainlessbot3699
      @brainlessbot3699 3 роки тому +1

      Then stop wasting

    • @davidbrisbane7206
      @davidbrisbane7206 3 роки тому

      Integration and differentiation is interesting to a point, but once you realise that computers can solve these questions, integration and differentiation becomes less interesting as a mathematical pursuit. Real & complex analysis are of course of great interest.

  • @AnatoArchives
    @AnatoArchives 3 роки тому +2

    First comment! Nice vid!

  • @AnatoArchives
    @AnatoArchives 3 роки тому +1

    Bruh why am I wasting my early teenager days with calculus.

    • @xCorvus7x
      @xCorvus7x 3 роки тому +1

      Are you?

    • @AnatoArchives
      @AnatoArchives 3 роки тому

      @@xCorvus7x YES, I AM!

    • @xCorvus7x
      @xCorvus7x 3 роки тому

      @@AnatoArchives
      Is this a JoJo reference?

  • @stepansamankov7129
    @stepansamankov7129 3 роки тому

    Wrong, if p is natural, then equality holds for p = 8, then a = 28, b = 6, if p belongs to the set of integers, then for p = -9, a = 45, b = 6