A050702 Nonprime numbers n such that n and n-reversed (<>n and no leading zeros) have the same number of prime factors and these prime factors (palindromes allowed here) are also reversals of each other.

26, 39, 62, 93, 143, 169, 187, 226, 286, 339, 341, 622, 682, 781, 933, 961, 1089, 1177, 1243, 1313, 1469, 1573, 1717, 2042, 2062, 2066, 2178, 2206, 2402, 2426, 2446, 2462, 2486, 2602, 2626, 2642, 3063, 3093, 3099, 3131, 3309, 3421, 3603, 3639, 3669, 3693, 3737, 3751, 3903, 3939, 3963, 4084

Offset

1,1

Comments

Prime factors counted without multiplicity. - Harvey P. Dale, Nov 29 2014

Links

Vincenzo Librandi, Table of n, a(n) for n = 1..1000 (corrected by Sean A. Irvine)

Example

Reversing 339 = 3*113 gives 933 = 3*311, both with two prime factors.

Mathematica

d[n_]:=IntegerDigits[n]; f[n_]:=First/@FactorInteger[n]; Select[Range[4100], !PrimeQ[#]&&Reverse/@d[f[#]]==d[f[x=FromDigits[Reverse[d[#]]]]]&&#!=x&](* Jayanta Basu, May 31 2013 *)
snpfQ[n_]:=Module[{pfn=Transpose[FactorInteger[n]][[1]], idn = IntegerDigits[ n], revn, pfrev, revpfrev}, revn = FromDigits[ Reverse[idn]]; pfrev=Transpose[ FactorInteger[ revn]][[1]]; revpfrev =FromDigits[Reverse[IntegerDigits[#]]]&/@pfrev; !PrimeQ[n]&& Last[ IntegerDigits[ n]] != 0&&revn!=n&&Length[pfn]==Length[pfrev]&&Union[pfn] == Union[ revpfrev]]; Select[ Range[ 4200], snpfQ] (* Harvey P. Dale, Nov 29 2014 *)

Crossrefs

Cf. A050699.

Sequence in context: A043147 A043927 A330701 * A105997 A354345 A075288

Adjacent sequences: A050699 A050700 A050701 * A050703 A050704 A050705

Keyword

nonn,base,nice

Author

Patrick De Geest, Aug 15 1999

Extensions

More terms from Naohiro Nomoto, Apr 03 2001

Corrected by Vincenzo Librandi, Feb 03 2014

Definition clarified by Harvey P. Dale, Nov 29 2014

Status

approved