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sigma(n) plus the n-th prime gives a square.
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%I #10 Aug 18 2019 04:33:11

%S 3,6,7,9,18,58,73,134,198,251,278,287,461,483,505,525,529,665,772,939,

%T 960,986,1291,1432,1463,1629,2115,2931,2991,3422,3888,4159,4231,4548,

%U 4715,6256,7867,9148,9474,9540,9958,10535,12035,12081,12223,12511

%N sigma(n) plus the n-th prime gives a square.

%H Amiram Eldar, <a href="/A114082/b114082.txt">Table of n, a(n) for n = 1..10000</a>

%e sigma(2115)+p(2115)=22201=149^2.

%t n=0; p=1; seq={}; Do[p = NextPrime[p]; n++; If[IntegerQ @ Sqrt [p + DivisorSigma[1, n]], AppendTo[seq, n]], {10^4}]; seq (* _Amiram Eldar_, Aug 18 2019 *)

%o (PARI) isok(n) = issquare(sigma(n) + prime(n)); \\ _Michel Marcus_, Jan 11 2014

%Y Cf. A114081, A114083.

%K nonn

%O 1,1

%A _Giovanni Resta_, Feb 13 2006