A064271 Numbers k such that sigma(k) - 2k is prime.
18, 20, 36, 100, 104, 196, 324, 392, 464, 576, 650, 784, 800, 900, 968, 1352, 1936, 1952, 2450, 2500, 3136, 4356, 4624, 5184, 6050, 6400, 9216, 10000, 10404, 10816, 12800, 13456, 14400, 15376, 15488, 16928, 18496, 20000, 20736, 23104, 26912
Offset: 1
Examples
n=100: sigma(100) - 2*100 = 17, a prime.
Links
- Harry J. Smith, Table of n, a(n) for n = 1..1000
Programs
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PARI
j=[]; for(n=1,50000,x=sigma(n)-2*n; if(x>0 && isprime(x),j=concat(j,n))); j
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PARI
{ n=0; for (m=1, 10^9, if (isprime(sigma(m) - 2*m), write("b064271.txt", n++, " ", m); if (n==1000, break)) ) } \\ Harry J. Smith, Sep 11 2009