OFFSET
1,1
COMMENTS
LINKS
Wikipedia, Polygonal number
EXAMPLE
15 is a term of this sequence, as it has 4 divisors and it can be represented in 4 different ways as an m-gonal number P(m,r) = r*((m-2)*r-(m-4))/2, with m>1, r>1, as following: 15 = P(15,2) = P(6,3) = P(3,5) = P(2,15).
PROG
(PARI) isok(k) = (k>1) && (sigma(k, 0) == sumdiv(2*k, d, (d>1) && (2*k/d + 2*d - 4) % (d-1) == 0));
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Daniel Suteu, Dec 22 2018
STATUS
approved