1,1

Theorem (I. N. Ianakiev): There are infinitely many such numbers. Proof: There are infinitely many square triangular numbers (A001110) and every (2t+1)-th of them is odd because A001110(0)=0, A001110(1)=1 and A001110(n)=34*a(n-1)-a(n-2)+2, for n>=2. Any sqrt(A001110(2t+1)) is odd (i. e. is in A005408) and can be written as p^2-q^2 because A005408(n)=A000290(n+1)-A000290(n). The unique values of p and q (p>q>0) for each sqrt(A001110(2t+1)) generate (when t>0) a unique Pythagorean triple with a unique hypotenuse (a=p^2-q^2, b=2pq, c=p^2+q^2). Therefore, there are infinitely many such hypotenuses squared.

Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

4 and 49 are in the sequence because 2^2=1^2+2*3/2 and 7^2=2^2+9*10/2

Cf. A000217, A000290, A182427.

Sequence in context: A343726 A202303 A350835 * A235001 A087055 A135556

Adjacent sequences: A214934 A214935 A214936 * A214938 A214939 A214940

Ivan N. Ianakiev, Jul 30 2012

approved