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A237912
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Smallest number m (not ending in a 0) such that m and the digit reversal of m have n prime factors (counted with multiplicity).
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2
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13, 15, 117, 126, 1386, 2576, 21708, 25515, 21168, 46848, 295245, 2937856, 6351048, 21989376, 217340928, 2154281472, 2196652032, 21120051456, 21122906112, 40915058688, 274148425728
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OFFSET
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1,1
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COMMENTS
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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
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LINKS
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Table of n, a(n) for n=1..21.
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EXAMPLE
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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.
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PROG
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(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
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CROSSREFS
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Cf. A004086.
Sequence in context: A318543 A302001 A109018 * A268664 A158697 A295151
Adjacent sequences: A237909 A237910 A237911 * A237913 A237914 A237915
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KEYWORD
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nonn,base,more
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AUTHOR
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Derek Orr, Feb 15 2014
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EXTENSIONS
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a(15)-a(21) from Giovanni Resta, Feb 23 2014
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STATUS
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approved
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