OFFSET
1,1
LINKS
Harvey P. Dale, Table of n, a(n) for n = 1..1000
EXAMPLE
532 is the 19th pentagonal number and 235=5*47 is semiprime.
MATHEMATICA
Select[PolygonalNumber[5, Range[200]], PrimeOmega[IntegerReverse[#]]==2&] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Jan 27 2019 *)
PROG
(Python)
from sympy import factorint
def ok(p): return sum(e for e in factorint(int(str(p)[::-1])).values()) == 2
print([p for p in (n*(3*n-1)//2 for n in range(143)) if ok(p)]) # Michael S. Branicky, Dec 22 2021
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Giovanni Resta, Jan 31 2006
STATUS
approved