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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
13, 15, 117, 126, 1386, 2576, 21708, 25515, 21168, 46848, 295245, 2937856, 6351048, 21989376, 217340928, 2154281472, 2196652032, 21120051456, 21122906112, 40915058688, 274148425728 (list; graph; refs; listen; history; text; internal format)
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.

a(22) > 10^12. - Giovanni Resta, Feb 23 2014

a(22) <= 21793707589632. - Chai Wah Wu, Dec 05 2014

LINKS

Table of n, a(n) for n=1..21.

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.

Sequence in context: A318543 A302001 A109018 * A268664 A158697 A295151

Adjacent sequences:  A237909 A237910 A237911 * A237913 A237914 A237915

KEYWORD

nonn,base,more

AUTHOR

Derek Orr, Feb 15 2014

EXTENSIONS

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

STATUS

approved

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Last modified June 24 20:32 EDT 2022. Contains 354830 sequences. (Running on oeis4.)