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Apart from initial term, composite numbers with property that concatenation of prime factors is a palindrome.
12

%I #20 Jan 03 2015 12:24:01

%S 1,4,8,9,16,25,27,32,39,49,64,69,81,119,121,125,128,129,159,219,243,

%T 249,256,259,329,339,343,403,429,469,507,512,625,669,679,729,795,1024,

%U 1207,1309,1329,1331,1533,1547,1587,1589,1703,2023,2048,2097,2187,2319

%N Apart from initial term, composite numbers with property that concatenation of prime factors is a palindrome.

%H Reinhard Zumkeller, <a href="/A046447/b046447.txt">Table of n, a(n) for n = 1..1000</a>

%e E.g., 81 = 3 * 3 * 3 * 3 -> 3333 is palindromic.

%t 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 *)

%o (Haskell)

%o a046447 n = a046447_list !! (n-1)

%o a046447_list = 1 : filter f [1..] where

%o f x = length ps > 1 && ps' == reverse ps'

%o where ps' = concatMap show ps; ps = a027746_row x

%o -- _Reinhard Zumkeller_, May 02 2014

%o (Python)

%o from sympy import factorint, isprime

%o A046447_list = [1]

%o for n in range(4,10**6):

%o ....if not isprime(n):

%o ........s = ''.join([str(p)*e for p,e in sorted(factorint(n).items())])

%o ........if s == s[::-1]:

%o ............A046447_list.append(n) # _Chai Wah Wu_, Jan 03 2015

%Y Cf. A046448, A046449, A046450.

%Y Cf. A027746, A018252, A136522, A002113.

%K nonn,nice,base

%O 1,2

%A _Patrick De Geest_, Jul 15 1998