OFFSET
1,1
COMMENTS
Smallest k such that A206400(k) = 2n-1.
EXAMPLE
a(4) = 15 because A206400(15) = 7 = 2*4 - 1. There are 7 composites of the form a^2+ 1 between the two primes 66^2+1 = 4357 and 74^2+1 = 5477.
MAPLE
n0:=500000:T:=array(1..n0):j:=0:i:=0:for m from 2 to n0 do:x:=m^2+1:if type (x, prime)=true then j:=j+1:T[j]:=i:i:=0:else i:=i+1:fi:od: for n from 1 to 50 do:ii:=0:for k from 1 to j while(ii=0) do:if T[k]=2*n-1 then ii:=1: printf(`%d, `, k):else fi:od:od:
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Lagneau, Oct 15 2012
STATUS
approved