Lemma 1. Let A≡2(mod3) be a positive integer. Then there exists a prime number p such that p≡2(mod3) and pα∣A where α is an odd integer.
Proof of lemma. Assume to the contrary that there is not such a prime number p. Therefore, if A=p1α1p2α2⋯pkαk is the prime factorization of A, we have two cases for pi's, 1≤i≤k, to consider:
Case 1. pi≡1(mod3)⇒piαi≡1(mod3).
Case 2. pi≡2(mod3) and 2∣αi⇒piαi≡(pi2)2αi≡1(mod3).
As a result, we must have A≡1(mod3) which is a contradiction. □
Lemma 2. Let p≡2(mod3) be a prime number. Show that {03,13,…,(p−1)3} is a complete residue system modulo p.
Proof of lemma. Obviously i3≡03(modp) iff i≡0(modp). Suppose that p∤i,j. Our goal is to show that i3≡j3(modp) iff i≡j(modp). One part of the proof is obvious. To prove the other part, suppose that p=3t+2. Then by Fermat's Little Theorem, we have i3t+1≡j3t+1≡1(modp). Hence, we have
i3ti≡i3t+1≡j3t+1≡(j3)tj≡i3tj(modp).
Since (i,p)=1, we get i≡j(modp). □
Now we are ready to solve the main problem. We claim that there is no such triple. Assume to the contrary that
a2+b2+c2=2013k(ab+bc+ca)
for some positive integer k.
First, without loss of generality we can suppose that a, b and c have no common factor, because if (a,b,c)=d>1, we can divide them by d to get a new triple with no common factor. We have (a+b+c)2=(2013k+2)(ab+bc+ca). 2013k+2≡2(mod3), so by lemma 1 there is some prime number p≡2(mod3) such that p2n+1∤2013k+2 (n≥0).
p2n+1∤2013k+2⇒p2n+1∣(a+b+c)2⇒p2n+2∣(a+b+c)2⇒p2n+2∣(2013k+2)(ab+bc+ca)⇒p∣ab+bc+ca.
As a result, p∣a+b+c and p∣ab+bc+ca. Hence
0≡ab+bc+ca≡ab+c(a+b)≡ab+c(−c)(modp)⇒ab≡c2(modp)⇒c3≡abc(modp).
By a similar argument, a3≡b3≡abc(modp), so by lemma 2 we deduce a≡b≡c(modp) and since p∣a+b+c and 3∤p, we find that p divides a, b and c, which contradicts our assumption that (a,b,c)=1. □