A046451 Semiprimes whose prime factors, when concatenated, yield a palindrome.

4, 9, 25, 39, 49, 69, 119, 121, 129, 159, 219, 249, 259, 329, 339, 403, 469, 669, 679, 1207, 1329, 1589, 1703, 2319, 2321, 2359, 2649, 2701, 3039, 3421, 3503, 3629, 3639, 3899, 4303, 4607, 4839, 5603, 5739, 6209, 6401, 6979, 7269, 7663, 7819, 8203, 8519

Offset: 1

Author

Patrick De Geest, Jul 15 1998

Keywords

• nonn
• base

Examples

			3899 = 7 * 557 -> 7557 is palindromic.
		

Links

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

Crossrefs

Cf. A046447.

Programs

Mathematica

d[n_] := IntegerDigits[n]; co[n_, k_] := Nest[Flatten[d[{#, n}]] &, n, k - 1]; Select[Range[8520], PrimeOmega[#] == 2 && Reverse[x = Flatten[d[co @@@ FactorInteger[#]]]] == x &] (* Jayanta Basu, Jun 26 2013 *)
Select[Range[9000],PrimeOmega[#]==2&&PalindromeQ[FromDigits[ Flatten[ IntegerDigits/@FactorInteger[#][[All,1]]]]]&] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Mar 19 2020 *)

Extensions

Edited by Charles R Greathouse IV, Apr 28 2010