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Odd composite numbers with only palindromic prime factors.
3

%I #17 Apr 07 2021 02:30:14

%S 9,15,21,25,27,33,35,45,49,55,63,75,77,81,99,105,121,125,135,147,165,

%T 175,189,225,231,243,245,275,297,303,315,343,363,375,385,393,405,441,

%U 453,495,505,525,539,543,567,573,605,625,655,675,693,707,729,735,755

%N Odd composite numbers with only palindromic prime factors.

%H Harvey P. Dale, <a href="/A046350/b046350.txt">Table of n, a(n) for n = 1..1000</a>

%t palQ[n_]:=Reverse[x=IntegerDigits[n]]==x; Select[Range[9,755,2],!PrimeQ[#]&&And@@palQ/@First/@FactorInteger[#]&] (* _Jayanta Basu_, Jun 05 2013 *)

%t Select[Range[9,800,2],CompositeQ[#]&&AllTrue[FactorInteger[#][[All,1]], PalindromeQ]&] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Mar 08 2018 *)

%o (Python)

%o from sympy import isprime, primefactors

%o def pal(n): s = str(n); return s == s[::-1]

%o def ok(n): return not isprime(n) and all(pal(f) for f in primefactors(n))

%o print(list(filter(ok, range(9, 756, 2)))) # _Michael S. Branicky_, Apr 06 2021

%Y Cf. A002385, A033620, A046349, A046351.

%K nonn,base

%O 1,1

%A _Patrick De Geest_, Jun 15 1998