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%I #11 Feb 28 2020 19:57:00
%S 1,9,15,25,59,444
%N Numbers n such that concatenation of first n primes, separated by zeros, is prime.
%C w_n= (p_1)0(p_2)0(p_3)...0(p_n) w_1=2 is prime(a_1=1), w_2, ..., w_8 are not prime and w_9 is prime (a_2=9),... a_n is the n-th term of w_n which is prime.
%C a(7) > 2615. - _Derek Orr_, Aug 12 2014
%e a(2)=9 because 2030507011013017019023, which is the concatenation of first 9 primes separated by zeros is prime.
%t Select[Range[450],PrimeQ[FromDigits[Flatten[IntegerDigits/@Riffle[ Prime[ Range[ #]],0]]]]&] (* _Harvey P. Dale_, Feb 28 2020 *)
%o (PARI)
%o p="";for(n=1,5000,p=concat(p,"0");p=concat(p,Str(prime(n)));if(ispseudoprime(eval(p)),print1(n,", "))) \\ _Derek Orr_, Aug 12 2014
%K base,hard,more,nonn
%O 1,2
%A _Farideh Firoozbakht_, May 03 2003