Generalized Pythagorean Triples

How many non-trivial solutions are there to the equation

a 2 + b 2 = c n \large a^2+b^2=c^n

for { a , b , c , n Z a , b , c 0 } \{ a, b, c, n \in \mathbb{Z}| a, b, c\neq0\} , and any n 3 n\geq3 ?

Bonus: Can you find a general formula?

Infinitely many. Has not been proven. Zero. Finitely many.

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6 solutions

Anirudh Sreekumar
Nov 13, 2018

Consider a primitive pythagorean triple \text{Consider a primitive pythagorean triple } ( a , b , c ) (a,b,c) such that \text{ such that}

a 2 + b 2 = c 2 multiplying throughout by c 2 n we get, ( a c n ) 2 + ( b c n ) 2 = ( c 2 ) n + 1 p 2 + q 2 = r n + 1 where p = a c n , q = b c n and r = c 2 since there are infinitely many Pythagorean triples and each one corresponds to a solution as shown above, there will be infinitely many solutions for each n. \begin{aligned}a^2+b^2&=c^2\\ \text{multiplying throughout by } c^{2n} \text{ we get,}\\ (ac^n)^2+(bc^n)^2&=(c^2)^{n+1} \\ p^2+q^2&=r^{n+1}\\ \text{where } p=ac^n,q=bc^n \text{ and } r=c^2\end{aligned}\\ \text{since there are infinitely many Pythagorean triples and each one corresponds to a solution as shown above,}\\ \text{ there will be infinitely many solutions for each n.}

One infinite family of solutions is ( a , b , c , n ) = ( 2 m , 2 m , 2 , 2 m + 1 ) (a,b,c,n) = (2^{m}, 2^{m}, 2, 2m + 1) for any integer m 1 m \ge 1 .

Mark Hennings
Nov 12, 2018

If we define integers a k , n , b k , n a_{k,n},b_{k,n} by setting a k , n + i b k , n = ( 2 + i ) k n a_{k,n} + ib_{k,n} \; = \; (2 + i)^{kn} then a k , n 2 + b k , n 2 = ( 2 + i ) 2 k n = 5 k n a_{k,n}^2 + b_{k,n}^2 \; = \; \big|(2+i)^{2kn}\big| \; = \; 5^{kn} and so ( a k , n , b k , n , 5 k , n ) \big(a_{k,n},b_{k,n},5^k,n\big) is a solution for any integers k 1 k \ge 1 and n 3 n \ge 3 . Thus we get infinitely many solutions for each value of n 3 n \ge 3 .

Andrei Li
Nov 14, 2018

There are infinitely many integer solutions for a 2 + b 2 = c n a^2+b^2=c^n , for any integer n 2 n\geq2 and { a , b , c Z a , b , c 0 } \{a,b,c\in\mathbb{Z}|a,b,c\neq0\} .

Note that I am not certain of the validity of this particular proof/formula. I will be grateful for any suggestions.

Consider any Gaussian integer z z . Perform the operation z n z^n , and recall that z n 2 = R e ( z n ) 2 + I m ( z n ) 2 , |z^n|^2=Re(z^n)^2+Im(z^n)^2, where R e ( z n ) Re(z^n) and I m ( z n ) Im(z^n) are respectively the real and imaginary coefficients of z n z^n . Now, the complex norm has a nice multiplicative property: z n = z n 2 |z^n|=|z|^{\frac{n}{2}} because z n = ( R e ( z n ) 2 + I m ( z n ) 2 ) n = ( R e ( z n ) 2 + I m ( z n ) 2 ) n 2 = ( R e ( z n ) 2 + I m ( z n ) 2 ) n = z n |z^n|=\sqrt{(Re(z^n)^2+Im(z^n)^2)^n}=(Re(z^n)^2+Im(z^n)^2)^{\frac{n}{2}}=(\sqrt{Re(z^n)^2+Im(z^n)^2})^n=|z|^n Substituting into the first equation, we obtain z n = R e ( z n ) 2 + I m ( z n ) 2 . |z|^{n}=Re(z^n)^2+Im(z^n)^2. As R e ( z n ) Re(z^n) , I m ( z n ) Im(z^n) , and z |z| are all positive integers, we have our proof.

Question: Does this formula include all possible integer solutions? What is the distribution of the solution amongst the integers?

David Vreken
Nov 13, 2018

An infinite family of solutions for a 2 + b 2 = c 4 a^2 + b^2 = c^4 is

a = p 4 6 p 2 q 2 + q 4 a = p^4 - 6p^2q^2 + q^4

b = 4 p 3 q 4 p q 3 b = 4p^3q - 4pq^3

c = p 2 + q 2 c = p^2 + q^2

for any integer p p and q q .

2^2 + 2^2 = 2^3

4^2 + 4^2 = 2^5

8^2 + 8^2 = 2^7

(2^x)^2 + (2^x)^2 = 2^(2x+1)

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