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

%I #17 Apr 13 2024 12:38:12

%S 2,3,5,6,10,11,13,15,18,19,20,24,26,28,31,33,41,50,51,53,56,59,60,63,

%T 70,76,78,80,82,83,86,89,90,93,95,99,101,102,103,104,111,116,122,123,

%U 131,132,142,143,144,145,146,148,149,150,152,153,154,155,156,161,163

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

%H Harry J. Smith, <a href="/A063451/b063451.txt">Table of n, a(n) for n = 1..1000</a>

%o (PARI) j=[]; for(n=1,350, if(isprime(n*sigma(n)-1),j=concat(j,n))); j

%o (PARI) n=0; for (m=1, 10^9, if (isprime(m*sigma(m) - 1), write("b063451.txt", n++, " ", m); if (n==1000, break))) \\ _Harry J. Smith_, Aug 21 2009

%K easy,nonn

%O 1,1

%A _Jason Earls_, Jul 25 2001