cp's OEIS Frontend

A237913 Smallest number m > 1 (not ending in a 0) such that m and the digit reversal of m have n prime factors (counted with multiplicity). Palindromes are included.

%I A237913 #23 May 22 2025 10:21:36
%S A237913 2,4,8,88,252,2576,21708,2112,4224,8448,44544,48384,2977792,21989376,
%T A237913 405504,4091904,441606144,405909504,886898688,677707776,4285005824,
%U A237913 276486684672,21128282112,633498894336,2701312131072,6739855589376,29142024192,65892155129856,4815463645184,445488555884544
%N A237913 Smallest number m > 1 (not ending in a 0) such that m and the digit reversal of m have n prime factors (counted with multiplicity). Palindromes are included.
%F A237913 a(n) = min{A076886(n+1), A237912(n)}
%e A237913 252 is the smallest number such that 252 and its reverse (also 252) have 5 prime factors (2*2*3*3*7). So, a(5) = 252.
%e A237913 2576 is the smallest number such that 2576 and its reverse (6752) have 6 prime factors (2*2*2*2*7*23 and 2*2*2*2*2*211, respectively). So a(6) = 2576.
%o A237913 (Python)
%o A237913 import sympy
%o A237913 from sympy import factorint
%o A237913 def rev(x):
%o A237913   rev = ''
%o A237913   for i in str(x):
%o A237913     rev = i + rev
%o A237913   return int(rev)
%o A237913 def RevFact(x):
%o A237913   n = 2
%o A237913   while n < 10**8:
%o A237913     if n % 10 != 0:
%o A237913       if sum(list(factorint(n).values())) == x:
%o A237913         if sum(list(factorint(rev(n)).values())) == x:
%o A237913           return n
%o A237913         else:
%o A237913           n += 1
%o A237913       else:
%o A237913         n += 1
%o A237913     else:
%o A237913       n += 1
%o A237913 x = 1
%o A237913 while x < 100:
%o A237913   print(RevFact(x))
%o A237913   x += 1
%Y A237913 Cf. A004086, A076886, A237912.
%K A237913 nonn,base
%O A237913 1,1
%A A237913 _Derek Orr_, Feb 15 2014
%E A237913 a(17)-a(21) from _Giovanni Resta_, Feb 23 2014
%E A237913 a(22)-a(30) from _Max Alekseyev_, Feb 08 2024