OFFSET
1,1
COMMENTS
Positive integers n such that n^2 = s^4*A147858(m)*A147858(k) for positive integers s and k<m. If n belongs to this sequence then so does n*s^2 for any positive integer s. Primitive elements of this sequence are given by A147856.
Euler proved that if n^2 = (x^4 - y^4)*(z^4 - t^4) then a,b,c (if n is even) or 4a,4b,4c (if n is odd) form a triple of integers with all pairwise sums and differences being squares, where a=(x^4+y^4)*(z^4+t^4)/2, b=(n^2+(2xyzt)^2)/2 and c=(n^2-(2xyzt)^2)/2. Note that a,b,c are pairwise distinct if and only if (x,y) and (z,t) are not proportional.
4*A196289(n) = 4*(n^9 - n) belong to this sequence since (4*(n^9 - n))^2 = ((n^4+2*n^2-1)^4 - (n^4-2*n^2-1)^4) * (n^4 - 1).
CROSSREFS
KEYWORD
nonn
AUTHOR
Max Alekseyev, Nov 17 2008, Nov 19 2008
STATUS
approved