login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A047820 Composite numbers that become prime after exactly 1 iteration of f(k) = sum of distinct prime factors of k. 4
4, 6, 8, 9, 10, 12, 16, 18, 20, 22, 24, 25, 27, 32, 34, 36, 40, 44, 48, 49, 50, 54, 58, 64, 68, 72, 80, 81, 82, 88, 96, 100, 108, 116, 118, 121, 125, 128, 136, 142, 144, 160, 162, 164, 165, 169, 176, 192, 200, 202, 210, 214, 216, 232, 236, 242, 243, 250, 256, 272 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

f(k) = sum of prime factors without multiplicity, so that f(1500) = 2+3+5 = 10.

The sequence is infinite because f(2^m * 5^s) = 2 + 5 = 7, for m,s >= 1. - Marius A. Burtea, Jan 21 2019

LINKS

Marius A. Burtea, Table of n, a(n) for n = 1..10122

MATHEMATICA

Select[ Range@280, (fi = FactorInteger@#; Plus @@ Last /@ fi > 1 && PrimeQ[Plus @@ First /@ fi]) &] (* Robert G. Wilson v, Dec 09 2005 *)

PROG

(MAGMA) [n:n in [1..300]| IsPrime(&+PrimeDivisors(n)) and not IsPrime(n) ] // Marius A. Burtea, Jan 21 2019

(PARI) is(n) = isprime(vecsum(factor(n)[, 1])) && !isprime(n) \\ David A. Corneth, Jan 21 2019

CROSSREFS

Cf. A000040, A002808, A008472.

Sequence in context: A141607 A071070 A275722 * A248807 A034878 A173328

Adjacent sequences:  A047817 A047818 A047819 * A047821 A047822 A047823

KEYWORD

nonn

AUTHOR

David W. Wilson

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.

License Agreements, Terms of Use, Privacy Policy. .

Last modified May 31 02:51 EDT 2020. Contains 334747 sequences. (Running on oeis4.)