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A118725 Chen primes for which the reversal is also a Chen prime. 3

%I #14 Jun 17 2020 02:08:31

%S 2,3,5,7,11,13,17,31,71,101,107,113,131,149,157,167,179,181,191,199,

%T 311,347,353,359,389,701,743,751,761,787,797,919,941,953,971,983,991,

%U 1009,1031,1061,1091,1097,1109,1151,1217,1229,1259,1283,1301,1409,1439

%N Chen primes for which the reversal is also a Chen prime.

%H Robert Israel, <a href="/A118725/b118725.txt">Table of n, a(n) for n = 1..10000</a>

%e 17 and its reversal 71 are both Chen primes.

%p revdigs:= proc(n) local L,k;

%p L:= convert(n,base,10);

%p add(L[-k]*10^(k-1),k=1..nops(L))

%p end proc:

%p filter:= proc(n) local r;

%p if not isprime(n) then return false fi;

%p r:= revdigs(n);

%p isprime(r) and numtheory:-bigomega(n+2) <= 2 and numtheory:-bigomega(r+2) <= 2

%p end proc:

%p select(filter, [2,seq(i,i=3..2000,2)]); # _Robert Israel_, Jun 16 2020

%t cpQ[n_]:=Module[{rev=FromDigits[Reverse[IntegerDigits[n]]]}, PrimeOmega[ n+2]<3 && PrimeQ[rev]&&PrimeOmega[rev+2]<3]; Select[Prime[ Range[ 400]], cpQ] (* _Harvey P. Dale_, Jul 17 2011 *)

%Y Cf. A109611.

%K base,nonn,less

%O 1,1

%A Luc Stevens (lms022(AT)yahoo.com), May 21 2006

%E Corrected by _Harvey P. Dale_, Jul 17 2011

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Last modified March 28 18:04 EDT 2024. Contains 371254 sequences. (Running on oeis4.)