A050702 - OEIS

%I #28 Mar 03 2023 20:27:13

%S 26,39,62,93,143,169,187,226,286,339,341,622,682,781,933,961,1089,

%T 1177,1243,1313,1469,1573,1717,2042,2062,2066,2178,2206,2402,2426,

%U 2446,2462,2486,2602,2626,2642,3063,3093,3099,3131,3309,3421,3603,3639,3669,3693,3737,3751,3903,3939,3963,4084

%N 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.

%C Prime factors counted without multiplicity. - _Harvey P. Dale_, Nov 29 2014

%H Vincenzo Librandi, <a href="/A050702/b050702.txt">Table of n, a(n) for n = 1..1000</a> (corrected by Sean A. Irvine)

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

%t 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 *)

%t 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 *)

%Y Cf. A050699.

%K nonn,base,nice

%O 1,1

%A _Patrick De Geest_, Aug 15 1999

%E More terms from _Naohiro Nomoto_, Apr 03 2001

%E Corrected by _Vincenzo Librandi_, Feb 03 2014

%E Definition clarified by _Harvey P. Dale_, Nov 29 2014