A180325 - OEIS

%I #10 Mar 30 2012 18:35:54

%S 1,6,9,31,16,18,40,89

%N a(n) = k is the smallest number such that n is the number of distinct primes dividing k! + 1.

%C A greedy inverse to A066856. - R. J. Mathar, Jan 21 2011

%e a(6) = 18 because the 6 distinct primes dividing 18! + 1 = 6402373705728001

%e   are {19, 23, 29, 61, 67, 123610951}.

%p with(numtheory):for n from 1 to 7 do:ind:=0:for k from 1 to 50 while(ind=0)

%p   do: x:=k!+1:y:=nops(factorset(x)):if y=n then ind:=1:printf(`%d, `,k):else fi:od:

%p   od:

%Y Cf. A038507.

%K nonn,hard

%O 1,2

%A _Michel Lagneau_, Jan 18 2011