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

1,1

Comments

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

Links

Table of n, a(n) for n=1..27.

Example

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.

Prog

(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

Crossrefs

Cf. A004086, A076886, A237913.

Sequence in context: A318543 A302001 A109018 * A268664 A158697 A393078

Adjacent sequences: A237909 A237910 A237911 * A237913 A237914 A237915

Keyword

nonn,base

Author

Derek Orr, Feb 15 2014

Extensions

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

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

Status

approved