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A089785 a(n) is the least prime(k) such that the concatenation of prime(k) and prime(k-n) is prime.

%I A089785 #7 Dec 13 2017 02:52:03
%S A089785 5,7,11,19,17,19,23,29,31,37,67,43,83,61,59,61,67,79,73,113,97,167,
%T A089785 107,101,103,149,109,137,167,163,137,179,149,163,257,179,277,173,251,
%U A089785 199,191,193,197,199,211,229,227,229,233,239,281,347,283,263,269,271,283
%N A089785 a(n) is the least prime(k) such that the concatenation of prime(k) and prime(k-n) is prime.
%H A089785 Robert Israel, <a href="/A089785/b089785.txt">Table of n, a(n) for n = 1..10000</a>
%e A089785 a(4) = 19 =prime(8) and prime(8) followed by prime(8-4) =197 is prime.
%p A089785 f:= proc(n) local p,k,q,d;
%p A089785  p:= ithprime(n);
%p A089785  for k from n+1 do
%p A089785    p:= nextprime(p);
%p A089785    q:= ithprime(k-n);
%p A089785    d:= ilog10(q)+1;
%p A089785    if isprime(p*10^d+q) then return p fi
%p A089785  od
%p A089785 end proc:
%p A089785 map(f, [$1..100]); # _Robert Israel_, Dec 13 2017
%Y A089785 Cf. A089782, A089783.
%K A089785 base,nonn
%O A089785 1,1
%A A089785 _Amarnath Murthy_, Nov 24 2003
%E A089785 More terms from _David Wasserman_, Oct 13 2005