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a(n) = x is the smallest number such that gcd(prime(x)-1,x-1) = n.
4

%I #13 May 05 2017 08:14:46

%S 1,3,4,13,36,19,120,33,46,11,78,37,560,239,496,1905,52,397,3250,221,

%T 778,573,2738,25,26,287,352,477,552,271,8588,449,5710,205,456,145,

%U 1000,4067,2536,761,14638,295,4344,2685,406,1151,612,3121,6616,551,868,469,20088

%N a(n) = x is the smallest number such that gcd(prime(x)-1,x-1) = n.

%H Ivan Neretin, <a href="/A084315/b084315.txt">Table of n, a(n) for n = 1..4000</a>

%F a(n) = Min{x; A084311(x)=n}.

%e In A084311, n=4 arises first at the 13th position, so a(4)=13.

%t f[x_] := GCD[Prime[x]-1, x-1] t=Table[0, {256}]; Do[s=f[n]; If[s<257&&t[[s]]==0, t[[s]]=n], {n, 1, 100000}]; t

%Y Cf. A000040, A084309-A084316.

%K nonn

%O 1,2

%A _Labos Elemer_, Jun 13 2003