

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
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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



