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A248746 a(n) is the index k of the greatest prime divisor A002313(k) of n^2 + 1.

%I A248746 #10 Feb 06 2021 21:52:52
%S A248746 1,2,2,4,3,6,2,3,7,13,9,5,4,22,15,26,5,3,20,39,4,12,8,51,31,60,10,18,
%T A248746 41,8,6,7,14,11,54,105,16,4,65,121,5,35,6,17,83,10,4,45,97,9,106,48,
%U A248746 29,209,11,221,3,59,133,28,138,66,38,25,155,294,43,6,174,5
%N A248746 a(n) is the index k of the greatest prime divisor A002313(k) of n^2 + 1.
%C A248746 a(n) is the number k such that A002313(k) = A014442(n).
%H A248746 Michel Lagneau, <a href="/A248746/b248746.txt">Table of n, a(n) for n = 1..5000</a>
%e A248746 a(5)=3 because A002313(3)=13 and 5^2+1 = 2*13 with A002313(3)= A014442(5).
%p A248746 with(numtheory):T:=array(1..50000):T[1]:=2:kk:=1:nn:=10^5:
%p A248746 for i from 1 to nn do:
%p A248746   p:=4*i+1:
%p A248746   if type(p,prime)=true
%p A248746   then
%p A248746     kk:=kk+1:T[kk]:=p:
%p A248746     else
%p A248746     fi:
%p A248746    od:
%p A248746      for k from 1 to 5000 do:ii:=0:
%p A248746       y:=factorset(k^2+1):n2:=nops(y):t:=y[n2]:
%p A248746         for l from 1 to kk while(ii=0)do :
%p A248746         if t=T[l]
%p A248746          then
%p A248746          printf(`%d, `,l):
%p A248746          else
%p A248746         fi:
%p A248746      od:
%p A248746     od:
%Y A248746 Cf. A014442 (greatest prime divisor of n^2+1), A002313 (primes congruent to 1 or 2 modulo 4).
%Y A248746 Cf. also A002522.
%K A248746 nonn
%O A248746 1,2
%A A248746 _Michel Lagneau_, Oct 13 2014