OFFSET
3,1
LINKS
Hiroaki Yamanouchi, Table of n, a(n) for n = 3..85
Eric Weisstein's World of Mathematics, Polygonal Number
EXAMPLE
For n=4, 81 is a square and sigma(81)=121 is also a square.
PROG
(PARI) a(n) = {k = 2; while(! ((p = k*((n-2)*k-(n-4))/2) && ispolygonal(sigma(p), n)), k++); k; }
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Marcus, Jun 22 2015
EXTENSIONS
a(23)-a(29) from Hiroaki Yamanouchi, Sep 26 2015
STATUS
approved