OFFSET
1,1
COMMENTS
EXAMPLE
2 is in the sequence because 2=1*2 is oblong, and sigma(2) = 1+2 = 3 = 2*3/2 is triangular.
MATHEMATICA
Select[2 Accumulate[Range@10000], MemberQ[Accumulate[Range@10000], DivisorSigma[1, #]] &] (* Michael De Vlieger, Mar 17 2015 *)
PROG
(PARI) {for (i=1, i=10^6, n=i*(i+1); if(ispolygonal(sigma(n), 3), print(n)))}
CROSSREFS
KEYWORD
nonn,changed
AUTHOR
Antonio Roldán, Mar 16 2015
STATUS
approved