OFFSET
1,8
COMMENTS
Sum of nonprime divisors of n that are less than n.
a(n) = 1 if n is prime or semiprime.
Up to 3*10^12, a(n) = n only for n = 42, 1316, and 131080256. In general, if p = 2^k-1 and q = 4^k-2*2^k-1 are two primes, then n = 2^(k-1)*p*q satisfies a(n) = n. This happens for k= 2, 3, 7, and 19, which give the aforementioned values and 3777871569031248714137. This property makes these values terms of A225028. - Giovanni Resta, May 03 2016
LINKS
Michel Lagneau, Table of n, a(n) for n = 1..10000
MAPLE
with(numtheory):for n from 1 to 100 do:x:=factorset(n):n1:=nops(x):s:=sum('x[i] ', 'i'=1..n1): s1:=sigma(n)-s-n: if type(n, prime)=true then printf(`%d, `, 1) else printf(`%d, `, s1):fi:od:
MATHEMATICA
Table[Plus@@Select[Divisors[n], #<n&&(!PrimeQ[#])&], {n, 1, 90}]
a[1]=0; a[n_]:= DivisorSigma[1, n] - n - Plus @@ First /@ FactorInteger @ n; Array[a, 100] (* Giovanni Resta, May 03 2016 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Lagneau, Jan 10 2013
STATUS
approved