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A165243 Primes obtained from other primes by pre-concatenating with 2. 7

%I

%S 23,211,223,229,241,271,283,2113,2131,2137,2179,2239,2251,2269,2281,

%T 2293,2311,2347,2383,2389,2467,2503,2521,2557,2593,2617,2647,2659,

%U 2677,2683,2719,2797,2857,2887,2953,2971,21013,21019,21031,21061,21163,21187

%N Primes obtained from other primes by pre-concatenating with 2.

%e The prime 2971 is from the prime 971 by pre-concatenating with 2.

%t Select[Map[FromDigits,

%t Map[Join[{2}, #] &, IntegerDigits[Prime[Range[200]]]]], PrimeQ[#] &] (* _Robert Price_, Mar 31 2019 *)

%o (PARI) terms(n) = my(i=0, cc=[]); forprime(p=1, , cc=eval(concat("2", Str(p))); if(ispseudoprime(cc), print1(cc, ", "); i++); if(i==n, break))

%o /* Print initial 40 terms as follows: */

%o terms(40) \\ _Felix Fröhlich_, Mar 31 2019

%Y Cf. A000040

%K base,nonn

%O 1,1

%A _Parthasarathy Nambi_, Sep 09 2009

%E 2269 inserted by _R. J. Mathar_, Sep 21 2009

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Last modified May 31 20:23 EDT 2020. Contains 334748 sequences. (Running on oeis4.)