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A046447
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Apart from initial term, composite numbers with property that concatenation of prime factors is a palindrome.
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12
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1, 4, 8, 9, 16, 25, 27, 32, 39, 49, 64, 69, 81, 119, 121, 125, 128, 129, 159, 219, 243, 249, 256, 259, 329, 339, 343, 403, 429, 469, 507, 512, 625, 669, 679, 729, 795, 1024, 1207, 1309, 1329, 1331, 1533, 1547, 1587, 1589, 1703, 2023, 2048, 2097, 2187, 2319
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OFFSET
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1,2
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LINKS
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EXAMPLE
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E.g., 81 = 3 * 3 * 3 * 3 -> 3333 is palindromic.
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MATHEMATICA
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concat[n_]:=Flatten[Table[IntegerDigits[First[n]], {Last[n]}]]; palQ[n_]:= Module[{x=Flatten[concat/@FactorInteger[n]]}, x==Reverse[x]&&!PrimeQ[n]]; Select[Range[2500], palQ] (* Harvey P. Dale, May 24 2011 *)
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PROG
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(Haskell)
a046447 n = a046447_list !! (n-1)
a046447_list = 1 : filter f [1..] where
f x = length ps > 1 && ps' == reverse ps'
where ps' = concatMap show ps; ps = a027746_row x
(Python)
from sympy import factorint, isprime
for n in range(4, 10**6):
....if not isprime(n):
........s = ''.join([str(p)*e for p, e in sorted(factorint(n).items())])
........if s == s[::-1]:
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CROSSREFS
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KEYWORD
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nonn,nice,base
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AUTHOR
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STATUS
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approved
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