Funnily enough I thought the Lemma that a+b+c always divides ab in a Pythagorean Triple was the cooler result. It's just a really neat looking simple relationship between a, b, and c. 🙂
@DeanCalhoun11 ай бұрын
I agree!
@miraj226411 ай бұрын
At 11:00, he's alluding to the case written out on the board. If a number x is odd, you can write it as 2n+1 for some n. So x^2 = 4n^2 + 4n + 1. If we take this mod 4, we see that an odd number squared mod 4 is always 1. Similarly, if y is even, you can write it as 2m for some m. So y^2 = 4m^2. If we take this mod 4, we get 0. We're dealing with Pythagorean triples here so we have a^2 + b^2 = c^2. If a and b are odd, that means the LHS is 2 mod 4. But the RHS can only be 1 if c is odd or 0 if c is even. Thus, a and b can't both be odd. So the only possible Pythagorean triples would be 2 evens (like 6^2 + 8^2 = 10^2) or an odd and an even (like 3^2 + 4^2 = 5^2).
@baerlauchstal11 ай бұрын
Yes, the parametrization method works I reckon. Let a = 2pq, b = p^2 - q^2, c = p^2 + q^2. Assume the triple is primitive. (Easy to show that if the claim holds for primitive triples it holds for all triples; and the parametrization doesn't necessarily work in any case if the triple isn't primitive.) Then p and q are coprime (because gcd(a,b,c) is obviously divisible by gcd(p,q)). It follows that p and (p + q) are coprime. Also p + q is odd; otherwise, each of a, b and c is even, contradicting the hypothesis of primitivity. Hence 2p and (p + q) are coprime. Finally, easy algebra gives a + b + c = 2p(p + q). Now, consider S = a^r + b^r + c^r = (2pq)^r + (p^2-q^2)^r + (p^2+q^2)^r, where r is odd. S is divisible by 2p, because (a) this is clearly true of (2pq)^r, and (b) the other two terms, when expanded, sum to 2p^(2r) + 2 (r choose 2) p^(2r-4)q^4 + ...; the power of p in each term is twice a positive odd number, and therefore greater than zero; hence 2p divides each term. And S is congruent modulo (p + q) to (2(-q)q)^r + (q^2 - q^2)^r + (q^2 + q^2)^r = -2^r q^(2r) + 2^r q^(2r) = 0. Hence S is divisible by both 2p and (p + q), and since these are coprime, by 2p(p + q). And we're done. This might be tidyable-up.
@baerlauchstal11 ай бұрын
Here's a tidying up of one bit. To show that S is divisible by 2p, we observe that this is true of (2pq)^r as before, and then show that the sum of the second and third terms is divisible by 2p^2. That's true because that sum is congruent, modulo 2p^2, to (-p^2-q^2)^r + (p^2+q^2)^r = -(p^2 + q^2)^r + (p^2 + q^2)^r = 0. Slightly neater?
@ivonovicsklrtweros11 ай бұрын
where is the backflip :(
@MathsMadeSimple10111 ай бұрын
I'm new here, what back flip?
@ezequielangelucci126311 ай бұрын
@@MathsMadeSimple101 He used to do backflips in the videos.
@ivonovicsklrtweros11 ай бұрын
@@MathsMadeSimple101 ı have watched all videos of this super amazing teacher, This guy used to always do backflips in his videos :(
@chrisbhag11 ай бұрын
Thanks for sharing professor! I Always enjoy your work! Wishing you all the best ❤🎉
@goodplacetostop297311 ай бұрын
17:19
@gregsarnecki758111 ай бұрын
‘Proof by hope’ - love it! It gives hope to all of us that hard maths can be overcome.
@pascalochem425611 ай бұрын
At 11:00, start with a^2+b^2=c^2, so a^2+b^2=c^2 (mod 2), so a+b=c (mod 2) since x^2=x (mod 2), so a+b-c=0 (mod 2), so a+b-c is even.
@DrR0BERT11 ай бұрын
I tackled this using the parameterization. It was tedious, and if I can streamline it I will. A few things I found is that a+b+c reduces to 2m^2+2mn = 2m (m+n). 2m and (m+n) are relatively prime. It's easy to show that 2m divides a^(2n+1)+b^(2n+1)+c^(2n+1). It gets tougher to show that m+n does as well.
@aadfg011 ай бұрын
This was my approach. Why is it tough? Let q = m+n. a = 2nm = -2n^2 mod q, b = 0 mod q, c = m^2+n^2 = 2n^2 mod q so the sum is (2n^2)^(2k+1) + (-2n^2)^(2k+1) = 0 mod q.
@DrR0BERT11 ай бұрын
@@aadfg0 Oh! I took the long way around.
@yossigil73911 ай бұрын
Can I suggest two problems for a video on variations Pythagorean triples? The natural question: What’s the parametrized form of a Pythagorean quartet? A variation is find all non trivial a,b,c,d\in\mathbf N such that a^2 + b^2 = c^2 + d^2, =>c^2-a^2 = d^2 - b^2 => (c-a)(c+a) = (d - b) (d +b). Denote x = c -a, y = c + a, z = d - b, u = d - b, v = d + b. => u v = xy , denote n = u v. Then an algorithm for generating all solutions is itertiating over all non primes and generating all distinct permutations of it. Not sure there is a parametrization though.
@prathikkannan332411 ай бұрын
Satisfying. This was a great problem solving solution.
@AnkhArcRod11 ай бұрын
At 5:27, though it is obvious that (a+b+c) cannot | a+b, it is not entirely obvious why gcd(a+b, a+b+c) = 1 (which, in general, is not true; eg. (6,8,10)). Thus, it would have been appropriate to write -ab(a+b) as -ab(a+b+c-c) = -ab(a+b+c) + abc. Thus, (a+b+c) should | abc. It turns out that indeed a+b+c |ab, but it would be wrong to jump to that conclusion at the step where we did.
@TheEternalVortex4211 ай бұрын
It doesn't really matter since a+b+c|ab => a+b+c+|ab(a+b). So you're right that a more complex condition could have been required but it turns out it wasn't.
@Qermaq11 ай бұрын
Nice. I thought about using the parameterization method you mentioned, but I doubt it would be a better proof. Would seem to me it would mostly parallel what you did here just with a bit more of a mess.
@bourhinorc142111 ай бұрын
Didn't you have a video "sum(a_k^n) is multiple of sum(a_k)?" Do someone have the link to it?
@vcvartak711111 ай бұрын
First time i understood after watching so many problems
@Timmmmartin11 ай бұрын
Without proving it, I'm pretty sure that by replacing b with -b in both the numerator and the denominator, another very similar result occurs, where b>a.
@師太滅絕11 ай бұрын
I always have a question, take 3,4,5 the triplets as example, can 3 or 4 be in another triplets? (Of course not 5, which can be in 5, 12, 13 triplets. Essentially, can the two smaller numbers of a P-triplets occur in another triplets (as the two smaller numbers) thanks
@stefanalecu953211 ай бұрын
There are no Pythagorean triangles in which the hypotenuse and one leg are the legs of another Pythagorean triangle (this is a corollary of Fermat's right triangle theorem), so that possibility is ruled out. You can have at most either that leg or the hypothenuse to be the leg of another triangle. You can find examples of distinct (a, b) pairs that satisfy the Pythagorean property and sum to the same c². If you have a primitive Pythagorean triple (where the gcd(a,b)=1) then you can generate it using two positive integers m>n>0 with m+n odd and gcd(m,n)=1, as Euclid has shown: (m²-n², 2mn, m²+n²). The hypothenuse is necessarily odd, so if you have (p²-q², 2pq, p²+q²) then it necessarily follows that you have m²+n²=p²-q² OR m²+n²=p²+q² (in which case there are an infinite number of triples with the same c²).
@yoav61311 ай бұрын
Very nice
@davidgillies62011 ай бұрын
If a, b, c are a primitive Pythagorean triple, one of a, b must be even and the other odd (if both odd c^2 = 2 mod 4, impossible; if both even a, b, c not primitive).
@maxxis403511 ай бұрын
I have a question for anyone really. So I have to determine if the following integral converge or diverge: from 0 to (pi)^2 of (1/[1- cos(sqrt(x)]), any help is appriciated.
@damadclown11 ай бұрын
Usually in these cases you can use a taylor series expansion around 0 (it's the only point where the integral can diverge): you'll get something like f(x) ~ 2/x which means the integral diverges. Hope it helps
@maxxis403511 ай бұрын
@@damadclown Ill look into it, thanks!
@baerlauchstal11 ай бұрын
@@maxxis4035 It's easier than that I think. Can you show that the integrand is bounded below by 2/x?
@divisix02411 ай бұрын
Remark: The identity a^3+b^3+c^3=(a+b+c)(a^2+b^2+c^2-ab-bc-ca)+3abc, which holds for all a,b,c without the need for Pythagorean triple, also necessitates the lemma.
@ismailozdas189411 ай бұрын
this theorem without pythagorean it will not be true because dividing 1^3 + 1^3 + 2^3 /1+1+2 is not integer
@divisix02411 ай бұрын
@@ismailozdas1894 Indeed, the lemma needs a,b,c to be a Pythagorean triple, only the identity does not. Notice how that definitely doesn’t imply a^3+b^3+c^3 is divisible by a+b+c.
@enpeacemusic19211 ай бұрын
Would be cool to derive an interesting identity that holds if and only if it’s an Pythagorean triple (that isn’t just the standard identity), since this one obviously is satisfied triples that aren’t Pythagorean (the identity is symmetric for all permutations of a, b, c while Pythagorean triples are symmetric for permutations of a and b)
@writerightmathnation948111 ай бұрын
Early misstatement of the denominator: “abc” instead of “a+b+c”. 😊
@charleyhoward459411 ай бұрын
who thinks of this stuff ? you've got to have a twisted mind to dance around to pt. where u can see all this out of the confusion