The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 56th year, we are closing in on 350,000 sequences, and we’ve crossed 9,700 citations (which often say “discovered thanks to the OEIS”).

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A046447 Apart from initial term, composite numbers with property that concatenation of prime factors is a palindrome. 12
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Reinhard Zumkeller, Table of n, a(n) for n = 1..1000 EXAMPLE E.g., 81 = 3 * 3 * 3 * 3 -> 3333 is palindromic. MATHEMATICA 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 *) PROG (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 -- Reinhard Zumkeller, May 02 2014 (Python) from sympy import factorint, isprime A046447_list = [1] 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]: ............A046447_list.append(n) # Chai Wah Wu, Jan 03 2015 CROSSREFS Cf. A046448, A046449, A046450. Cf. A027746, A018252, A136522, A002113. Sequence in context: A075309 A175031 A052054 * A087244 A080062 A249125 Adjacent sequences:  A046444 A046445 A046446 * A046448 A046449 A046450 KEYWORD nonn,nice,base AUTHOR Patrick De Geest, Jul 15 1998 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified December 6 06:14 EST 2021. Contains 349563 sequences. (Running on oeis4.)