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URL: https://oeis.org/A155101

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A155101
Nonprimes k such that the sum of prime factors of composite(k) is an even composite.
0
1, 4, 8, 9, 10, 12, 15, 20, 21, 24, 26, 28, 33, 34, 35, 38, 40, 42, 45, 46, 49, 50, 51, 55, 56, 58, 60, 63, 66, 68, 70, 72, 74, 75, 80, 81, 84, 88, 90, 92, 96, 98, 99, 100, 102, 104, 105, 106, 108, 110, 117, 118, 119, 121, 123, 124, 128, 129, 130, 133, 136, 140, 142, 143
OFFSET
1,2
EXAMPLE
1 (nonprime) is a term because composite(1) = 4 = 2*2 and 2 + 2 = 4 is an even composite;
4 (nonprime) is a term because composite(4) = 9 = 3*3 and 3 + 3 = 6 is an even composite;
8 (nonprime) is a term because composite(8) = 15 = 3*5 and 3 + 5 = 8 is an even composite.
MAPLE
pss := proc(n) local a, d ; a := 0 ; for d in ifactors(n)[2] do a := a+ op(2, d)*op(1, d) ; end do: a ; end proc:
isA002808 := proc(n) n >= 4 and not isprime(n) ; end proc:
A002808 := proc(n) option remember ; local a; if n = 1 then 4; else for a from procname(n-1)+1 do if not isprime(a) then return a; end if; end do; end if; end proc:
for n from 1 to 400 do if not isprime(n) then c := pss(A002808(n)) ; if type(c, 'even') and isA002808(c) then printf("%d, ", n) ; end if; end if; end do: # R. J. Mathar, May 10 2010
CROSSREFS
KEYWORD
nonn
AUTHOR
EXTENSIONS
Corrected (44 removed, 80 inserted) by R. J. Mathar, May 10 2010
STATUS
approved