A115709 Pentagonal numbers (A000326) whose digit reversal is a semiprime (A001358).

12, 22, 51, 176, 330, 532, 590, 715, 782, 925, 1162, 1247, 1335, 1426, 1717, 3151, 3290, 4187, 5551, 7107, 7315, 7957, 10542, 10795, 11051, 11837, 12376, 14950, 15251, 15555, 15862, 16172, 17120, 19097, 19780, 20126, 22265, 24130, 24512, 26467, 26867, 30175

Offset: 1

Author

Giovanni Resta, Jan 31 2006

Keywords

• nonn
• base

Examples

			532 is the 19th pentagonal number and 235=5*47 is semiprime.
		

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A000326, A001358, A115708.

Programs

Mathematica

Select[PolygonalNumber[5,Range[200]],PrimeOmega[IntegerReverse[#]]==2&] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Jan 27 2019 *)

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