A115708 Semiprimes (A001358) whose digit reversal is a pentagonal number (A000326).

10, 15, 21, 22, 235, 287, 517, 529, 671, 1513, 1555, 2611, 5137, 5331, 6241, 7017, 7171, 7421, 7597, 7814, 10078, 10913, 15011, 15094, 15214, 15251, 20395, 20603, 21542, 23129, 24501, 24873, 25157, 26851, 27161, 51998, 53297, 54989, 55551

Offset

1,1

Links

Harvey P. Dale, Table of n, a(n) for n = 1..100

Example

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

Mathematica

Select[Range[60000], PrimeOmega[#]==2&&IntegerQ[(1+Sqrt[1+24* IntegerReverse[#]])/ 6]&] (* Harvey P. Dale, Apr 27 2022 *)

Crossrefs

Cf. A000326, A001358, A115709.

Sequence in context: A105156 A108614 A336548 * A068992 A325901 A098564

Adjacent sequences: A115705 A115706 A115707 * A115709 A115710 A115711

Keyword

nonn,base

Author

Giovanni Resta, Jan 31 2006

Status

approved