OFFSET
1,1
COMMENTS
For these semiprimes k*(5*k-3)/2, the corresponding k are listed in A114517.
LINKS
Colin Barker, Table of n, a(n) for n = 1..1000
EXAMPLE
The heptagonal number 34 is in the sequence because 34 = 2 * 17.
MATHEMATICA
a={}; Do[If[PrimeOmega[n (5 n - 3) / 2]==2, AppendTo[a, n(5 n - 3) / 2]], {n, 1, 200}]; a (* Vincenzo Librandi, Jul 04 2015 *)
Select[PolygonalNumber[7, Range[300]], PrimeOmega[#]==2&] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Jun 07 2021 *)
PROG
(PARI)
pg(m, n) = (n^2*(m-2)-n*(m-4))/2 \\ n-th m-gonal number
select(n->bigomega(n)==2, vector(2000, n, pg(7, n)))
(Magma) IsSemiprime:=func<n | &+[d[2]: d in Factorization(n)] eq 2>; [s: n in [2..300] | IsSemiprime(s) where s is n*(5*n-3) div 2]; // Vincenzo Librandi, Jul 04 2015
CROSSREFS
KEYWORD
nonn
AUTHOR
Colin Barker, Jul 03 2015
STATUS
approved