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Numbers k such that sigma(2*k-1) is a prime p.
11

%I #16 Nov 11 2024 01:57:29

%S 5,13,145,365,841,1201,1741,2521,3961,5101,7813,8581,13945,14281,

%T 14965,41761,42925,73345,139921,229165,245701,265721,276025,289561,

%U 298765,341965,351961,353641,367225,414961,595141,601705,676285,697381,711625,740545,942565

%N Numbers k such that sigma(2*k-1) is a prime p.

%C Supersequence of A247789.

%C Corresponding values of primes p for a(n) are A008438(a(n)-1) or A247837.

%H Martin Møller Skarbiniks Pedersen, <a href="/A247820/b247820.txt">Table of n, a(n) for n = 1..1000</a>

%e Number 13 is in sequence because sigma(2*13-1) = sigma(25) = 31 (prime).

%o (Magma) [n: n in [1..10000000] | IsPrime(SumOfDivisors(2*n-1))];

%o (PARI) for(n=1,10^7,if(isprime(sigma(2*n-1)),print1(n,", "))) \\ _Derek Orr_, Sep 25 2014

%Y Cf. A000203, A008438, A247789, A247837.

%K nonn

%O 1,1

%A _Jaroslav Krizek_, Sep 24 2014