OFFSET
1,2
PROG
(PARI) is(n)=my(t=floor((sqrt(8*n^2)-1)/2)+1); t=t*(t+1)/2-n^2; my(tt=floor((sqrt(8*t)-1)/2)+1); (tt*(tt+1)/2==t)
CROSSREFS
KEYWORD
nonn
AUTHOR
Ralf Stephan, Oct 08 2013
STATUS
approved