This site is supported by donations to The OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A231587 Numbers n such that sigma(sigma(n)) - sigma(n) + 1 is prime. 2
 2, 4, 5, 6, 9, 11, 12, 13, 14, 15, 16, 19, 23, 24, 25, 29, 34, 37, 38, 42, 43, 48, 53, 54, 56, 57, 59, 60, 62, 63, 64, 67, 69, 73, 74, 75, 76, 77, 78, 79, 80, 81, 84, 85, 87, 88, 90, 91, 92, 95, 103, 104, 105, 107, 109, 111, 112, 113, 116, 118, 120, 122, 123, 124 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS K. D. Bajpai, Table of n, a(n) for n = 1..10000 EXAMPLE a(3)= 5: sigma(sigma(5))-sigma(5)+1= 12-6+1= 7, which is prime. a(10)= 15: sigma(sigma(15))-sigma(15)+1= 60-24+1= 37, which is prime. MAPLE 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); CROSSREFS Cf. A000203 (a(n): sigma(n)). Cf. A051027 (a(n): sigma(sigma(n)). Cf. A228567 (primes: sigma(sigma(n))-sigma(n)). Sequence in context: A285163 A015834 A252041 * A050011 A030303 A047315 Adjacent sequences:  A231584 A231585 A231586 * A231588 A231589 A231590 KEYWORD nonn AUTHOR K. D. Bajpai, Nov 11 2013 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified May 26 17:17 EDT 2019. Contains 323597 sequences. (Running on oeis4.)