A237912 Smallest number m (not ending in a 0) such that m and its digit reversal A004086(m) both have n prime factors (counted with multiplicity).

13, 15, 117, 126, 1386, 2576, 21708, 25515, 21168, 46848, 295245, 2937856, 6351048, 21989376, 217340928, 2154281472, 2196652032, 21120051456, 21122906112, 40915058688, 274148425728, 2150086519296, 2707602702336, 6167442456576, 21907217055744, 29798871072768, 420127895977984

Offset: 1

Author

Derek Orr, Feb 15 2014

Keywords

• nonn
• base

Comments

Palindromes are not included in this sequence since the reverse of a palindrome is the same number. See A076886 and A237913.

Examples

			13 and 31 are both prime so a(1) = 13.
15 and 51 have two prime factors (3*5 and 3*17 respectively), so a(2) = 15.
		

Crossrefs

Cf. A004086, A076886, A237913.

Programs

Python

import sympy
from sympy import factorint
def rev(x):
  rev = ''
  for i in str(x):
    rev = i + rev
  return int(rev)
def RevFact(x):
  n = 1
  while n < 10**8:
    if rev(n) != n:
      if n % 10 != 0:
        if sum(list(factorint(n).values())) == x:
          if sum(list(factorint(rev(n)).values())) == x:
            return n
          else:
            n += 1
        else:
          n += 1
      else:
        n += 1
    else:
      n += 1
x = 1
while x < 100:
  print(RevFact(x))
  x += 1

Extensions

a(15)-a(21) from Giovanni Resta, Feb 23 2014

a(22)-a(27) from Max Alekseyev, Feb 07 2024