A231587 - OEIS

%I #6 Nov 11 2013 14:11:35

%S 2,4,5,6,9,11,12,13,14,15,16,19,23,24,25,29,34,37,38,42,43,48,53,54,

%T 56,57,59,60,62,63,64,67,69,73,74,75,76,77,78,79,80,81,84,85,87,88,90,

%U 91,92,95,103,104,105,107,109,111,112,113,116,118,120,122,123,124

%N Numbers n such that sigma(sigma(n)) - sigma(n) + 1 is prime.

%H K. D. Bajpai, <a href="/A231587/b231587.txt">Table of n, a(n) for n = 1..10000</a>

%e a(3)= 5: sigma(sigma(5))-sigma(5)+1= 12-6+1= 7, which is prime.

%e a(10)= 15: sigma(sigma(15))-sigma(15)+1= 60-24+1= 37, which is prime.

%p with(numtheory):KD := proc() local a; a:= sigma(sigma(n))-sigma(n)+1; if isprime(a) then RETURN (n); fi; end: seq(KD(),n=1..500);

%Y Cf. A000203 (a(n): sigma(n)).

%Y Cf. A051027 (a(n): sigma(sigma(n)).

%Y Cf. A228567 (primes: sigma(sigma(n))-sigma(n)).

%K nonn

%O 1,1

%A _K. D. Bajpai_, Nov 11 2013