OFFSET
1,2
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..72
EXAMPLE
441 is in the sequence because 441 = 21^2 is square number, and sigma(441) = 441 + 147 + 63 + 49 + 21 + 9 + 7 + 3 + 1 = 741 = 38*39/2 is triangular number.
MATHEMATICA
t = Accumulate[Range@ 10000]; Select[Range[10000]^2, MemberQ[t, DivisorSigma[1, #]] &] (* Michael De Vlieger, Mar 17 2015 *)
Select[Range[500000]^2, OddQ[Sqrt[8DivisorSigma[1, #]+1]]&] (* Harvey P. Dale, Feb 25 2017 *)
PROG
(PARI) {for(i=1, 10^6, n=i*i; if(ispolygonal(sigma(n), 3), print1(n, ", ")))}
CROSSREFS
KEYWORD
nonn
AUTHOR
Antonio Roldán, Mar 16 2015
STATUS
approved