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%I #13 Jun 08 2018 11:29:17
%S 2,4,38,16,170,72,446,58,512,282,178,148,758,856,836,1592,1712,388,
%T 1906,2606,2034,1918,656,5924,1648,13082,652,1514,2758,10922,5758,
%U 18986,6764,10570,20918,4936,8188,5842,4094,30710,15212,11482,57932,14626,5624,36232,16018,57874
%N a(n) is twice the smallest k such that A051686(k) = prime(n).
%C The sequence is based on the first 50000 terms of A051686, in which the first 54 primes (2,3,...,251) appear along with 19 others, the largest of which is A051686(37976) = 823.
%e The 25th term in this sequence is 1648. This means that prime(25) = 97 arises in A051686 as A051686(1648/2) = A051686(824). Thus, 1648 is the first term in the sequence {..., 2k, ...} = {1648, 1798, 4108, ...} with the property that 2k*97 + 1 = 194k + 1 is also a prime, moreover the smallest one: 159857.
%o (PARI) a051686(n) = my(p=2); while(!isprime(2*n*p+1), p = nextprime(p+1)); p;
%o a(n) = my(k=1); while(a051686(k) != prime(n), k++); 2*k; \\ _Michel Marcus_, Jun 08 2018
%Y Cf. A005384, A051644-A051654, A051860, A051861.
%K nonn
%O 1,1
%A _Labos Elemer_
%E More terms from _Michel Marcus_, Jun 08 2018