A050701 If composite k and its reverse are different and have same number of prime factors, then the larger of them is a term of the sequence.

51, 62, 85, 93, 94, 221, 302, 321, 341, 381, 413, 442, 492, 493, 502, 511, 513, 514, 522, 524, 533, 534, 551, 553, 561, 562, 574, 581, 582, 604, 605, 621, 622, 623, 642, 663, 682, 685, 705, 711, 723, 734, 741, 766, 771, 781, 794, 805, 814, 817

Offset: 1

Author

Patrick De Geest, Aug 15 1999

Keywords

• nonn
• base

Examples

			a(n)=341 -> reverse(a(n)) = 143 gives the pair (143,341) of which only the larger value 341 is retained.
		

Crossrefs

Cf. A004086, A050699, A050700.

Programs

Mathematica

rev[n_]:=FromDigits[Reverse[IntegerDigits[n]]]; Select[Range[825],!PrimeQ[#]&&PrimeOmega[#]==PrimeOmega[x=rev[#]]&&#>x&] (* Jayanta Basu, May 31 2013 *)

PARI

isok(m) = my(k=fromdigits(Vecrev(digits(m)))); (m%10) && !isprime(m) && (m>k) && (bigomega(k) == bigomega(m)); \\ Michel Marcus, Aug 18 2021

Extensions

Revised by Editors of OEIS, Oct 19 2019

Incorrect 394 and 523 removed and name clarified by Sean A. Irvine, Aug 17 2021