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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. 2
2, 4, 8, 88, 252, 2576, 21708, 2112, 4224, 8448, 44544, 48384, 2977792, 21989376, 405504, 4091904, 441606144, 405909504, 886898688, 677707776, 4285005824 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

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

FORMULA

a(n) = min{A076886(n+1), A237912(n)}

EXAMPLE

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.

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.

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 = 2

..while n < 10**8:

....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

x = 1

while x < 100:

..print(RevFact(x))

..x += 1

CROSSREFS

Cf. A004086, A076886, A237912.

Sequence in context: A088114 A348050 A239697 * A076886 A309565 A046385

Adjacent sequences:  A237910 A237911 A237912 * A237914 A237915 A237916

KEYWORD

nonn,base,more

AUTHOR

Derek Orr, Feb 15 2014

EXTENSIONS

a(17)-a(21) from Giovanni Resta, Feb 23 2014

STATUS

approved

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Last modified August 8 14:39 EDT 2022. Contains 356009 sequences. (Running on oeis4.)