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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. 3

%I

%S 5,7,11,19,17,19,23,29,31,37,67,43,83,61,59,61,67,79,73,113,97,167,

%T 107,101,103,149,109,137,167,163,137,179,149,163,257,179,277,173,251,

%U 199,191,193,197,199,211,229,227,229,233,239,281,347,283,263,269,271,283

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

%H Robert Israel, <a href="/A089785/b089785.txt">Table of n, a(n) for n = 1..10000</a>

%e a(4) = 19 =prime(8) and prime(8) followed by prime(8-4) =197 is prime.

%p f:= proc(n) local p,k,q,d;

%p p:= ithprime(n);

%p for k from n+1 do

%p p:= nextprime(p);

%p q:= ithprime(k-n);

%p d:= ilog10(q)+1;

%p if isprime(p*10^d+q) then return p fi

%p od

%p end proc:

%p map(f, [$1..100]); # _Robert Israel_, Dec 13 2017

%Y Cf. A089782, A089783.

%K base,nonn

%O 1,1

%A _Amarnath Murthy_, Nov 24 2003

%E More terms from _David Wasserman_, Oct 13 2005

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Last modified May 13 23:41 EDT 2021. Contains 343868 sequences. (Running on oeis4.)