cp's OEIS Frontend

A122490 Least number k>1 such that k+10^n is a symmetric prime with symmetric digits (i.e. such that k+10^n is in A007500).

%I A122490 #13 Nov 09 2017 14:18:07
%S A122490 3,7,9,7,49,33,169,7,7,207,237,91,313,261,273,79,49,2901,51,441,193,9,
%T A122490 531,289,1141,67,909,331,753,2613,657,49,4459,603,1531,849,2049,259,
%U A122490 649,2119,1483,63,6747,519,3133,937,1159,1999,6921,2949,613,4137,1977,31,483,883,8553,12117,1009,4347,733
%N A122490 Least number k>1 such that k+10^n is a symmetric prime with symmetric digits (i.e. such that k+10^n is in A007500).
%H A122490 Robert Israel, <a href="/A122490/b122490.txt">Table of n, a(n) for n = 1..722</a>
%e A122490 3+10^1=13, 13 and 31 are symmetric primes with symmetric digits;
%e A122490 7+10^2=107, 107 and 701 are symmetric primes with symmetric digits;
%e A122490 9+10^3=1009, 1009 and 9001 are symmetric primes with symmetric digits.
%p A122490 revdigs:= proc(n) local L,i;
%p A122490   L:= convert(n,base,10);
%p A122490   add(L[-i]*10^(i-1),i=1..nops(L))
%p A122490 end proc:
%p A122490 f:= proc(n) local k,m;
%p A122490    for k from 3 by 2 do
%p A122490      m:= k+10^n;
%p A122490      if isprime(m) and isprime(revdigs(m)) then return k fi
%p A122490    od
%p A122490 end proc:
%p A122490 map(f, [$1..100]); # _Robert Israel_, Nov 09 2017
%Y A122490 Cf. A007500
%K A122490 nonn,base
%O A122490 1,1
%A A122490 _Pierre CAMI_, Sep 16 2006
%E A122490 Definition clarified, a(28) corrected, and more terms added by _Robert Israel_, Nov 09 2017