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A340487 a(n) is the least prime that is the concatenation of two n-digit primes, or 0 if there are none.

%I A340487 #53 Jan 13 2021 10:39:58
%S A340487 23,1117,101107,10091021,1000710181,100003100129,10000031000171,
%T A340487 1000001910000349,100000007100000541,10000000071000000349,
%U A340487 1000000001910000000319,100000000003100000000063,10000000000391000000000903,1000000000003710000000000259,100000000000031100000000000403
%N A340487 a(n) is the least prime that is the concatenation of two n-digit primes, or 0 if there are none.
%C A340487 Conjecture: a(n) > 0 and the first n digits of a(n) = A003617(n). - _Chai Wah Wu_, Jan 13 2021
%H A340487 Chai Wah Wu, <a href="/A340487/b340487.txt">Table of n, a(n) for n = 1..500</a> (n = 1..380 from Robert Israel)
%e A340487 a(3) = 101107 is prime and the concatenation of the two 3-digit primes 101 and 107.
%p A340487 f:= proc(d) local P,a,b;
%p A340487   a:= prevprime(10^(d-1));
%p A340487   do
%p A340487     a:= nextprime(a);
%p A340487     if a > 10^d then return FAIL fi;
%p A340487     b:= prevprime(10^(d-1));
%p A340487     do
%p A340487       b:= nextprime(b);
%p A340487       if b > 10^d then break fi;
%p A340487       if isprime(10^d*a+b) then return 10^d*a+b fi;
%p A340487   od od:
%p A340487   FAIL
%p A340487 end proc:
%p A340487 f(1):= 23:
%p A340487 map(f, [$1..20]);
%Y A340487 Cf. A003617.
%K A340487 nonn,base
%O A340487 1,1
%A A340487 _J. M. Bergot_ and _Robert Israel_, Jan 10 2021