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A322982 If n is a noncomposite, then a(n) = 2*n - 1, otherwise a(n) = A032742(n), the largest proper divisor of n.

%I A322982 #15 Jan 04 2019 09:04:36
%S A322982 1,3,5,2,9,3,13,4,3,5,21,6,25,7,5,8,33,9,37,10,7,11,45,12,5,13,9,14,
%T A322982 57,15,61,16,11,17,7,18,73,19,13,20,81,21,85,22,15,23,93,24,7,25,17,
%U A322982 26,105,27,11,28,19,29,117,30,121,31,21,32,13,33,133,34,23,35,141,36,145,37,25,38,11,39,157,40,27,41,165,42,17,43,29
%N A322982 If n is a noncomposite, then a(n) = 2*n - 1, otherwise a(n) = A032742(n), the largest proper divisor of n.
%H A322982 Antti Karttunen, <a href="/A322982/b322982.txt">Table of n, a(n) for n = 1..20000</a>
%F A322982 If A010051(n) == 1, a(n) = 2*n - 1, otherwise a(n) = A032742(n) = n/A020639(n).
%t A322982 Array[If[CompositeQ@ #, Divisors[#][[-2]], 2 # - 1] &, 87] (* _Michael De Vlieger_, Jan 03 2019 *)
%o A322982 (PARI) A322982(n) = if((1==n)||isprime(n),n+n-1,n/vecmin(factor(n)[,1]));
%Y A322982 Cf. A010051, A020639, A032742, A087717.
%K A322982 nonn
%O A322982 1,2
%A A322982 _Antti Karttunen_, Jan 03 2019