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A144719 Semiprimes whose factors are decimal palindromes when concatenated, omitting multiples of primes less than 11. 2

%I #6 Sep 12 2020 13:23:51

%S 121,403,1207,1243,1703,2321,2701,3421,3503,3629,4043,4061,4069,4121,

%T 4247,4303,4309,4607,5603,6109,6401,7663,8203,8921,9329,9703,9707,

%U 9943,10001,10021,10187,10201,12223,12283,12709,13129,13801,14921,15007

%N Semiprimes whose factors are decimal palindromes when concatenated, omitting multiples of primes less than 11.

%C Presumed infinite.

%H Harvey P. Dale, <a href="/A144719/b144719.txt">Table of n, a(n) for n = 1..1000</a>

%e 121 = 11 * 11; 1111 is a palindrome.

%e 4309 = 139 * 31; 13931 is a palindrome.

%t spdpQ[n_]:=Module[{fi=FactorInteger[n][[All,1]]},PrimeOmega[n]==2&&Min[ fi]>10 &&AnyTrue[{FromDigits[Flatten[IntegerDigits/@fi]],FromDigits[ Flatten[ IntegerDigits/@Reverse[fi]]]},PalindromeQ]]; Select[Range[ 15100],spdpQ] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Sep 12 2020 *)

%Y Cf. A002113, A116935

%K base,easy,nonn

%O 1,1

%A _Reikku Kulon_, Sep 19 2008

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Last modified May 19 00:35 EDT 2024. Contains 372666 sequences. (Running on oeis4.)