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 A237912 Smallest number m (not ending in a 0) such that m and the digit reversal of m have n prime factors (counted with multiplicity). 2

%I

%S 13,15,117,126,1386,2576,21708,25515,21168,46848,295245,2937856,

%T 6351048,21989376,217340928,2154281472,2196652032,21120051456,

%U 21122906112,40915058688,274148425728

%N Smallest number m (not ending in a 0) such that m and the digit reversal of m have n prime factors (counted with multiplicity).

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

%C a(22) > 10^12. - _Giovanni Resta_, Feb 23 2014

%C a(22) <= 21793707589632. - _Chai Wah Wu_, Dec 05 2014

%e 13 and 31 are both prime so a(1) = 13.

%e 15 and 51 have two prime factors (3*5 and 3*17 respectively), so a(2) = 15.

%o (Python)

%o import sympy

%o from sympy import factorint

%o def rev(x):

%o ..rev = ''

%o ..for i in str(x):

%o ....rev = i + rev

%o ..return int(rev)

%o def RevFact(x):

%o ..n = 1

%o ..while n < 10**8:

%o ....if rev(n) != n:

%o ......if n % 10 != 0:

%o ........if sum(list(factorint(n).values())) == x:

%o ..........if sum(list(factorint(rev(n)).values())) == x:

%o ............return n

%o ..........else:

%o ............n += 1

%o ........else:

%o ..........n += 1

%o ......else:

%o ........n += 1

%o ....else:

%o ......n += 1

%o x = 1

%o while x < 100:

%o ..print(RevFact(x))

%o ..x += 1

%Y Cf. A004086.

%K nonn,base,more

%O 1,1

%A _Derek Orr_, Feb 15 2014

%E a(15)-a(21) from _Giovanni Resta_, Feb 23 2014

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Last modified August 14 12:15 EDT 2022. Contains 356116 sequences. (Running on oeis4.)