OFFSET
1,1
REFERENCES
Glenn James and Robert C. James, Mathematics Dictionary, Princeton, N.J.: D. Van Nostrand Co., Inc., 1959; page 154 (factor of an integer).
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
FORMULA
Factor n into its distinct divisors, ignore n and 1, add the divisors and divide by the number of divisors. If s(d) / c(d) [sum divided by count] is prime, add to sequence.
EXAMPLE
a(1)= 8 because its factors are 8, 1, 2, 4. Ignoring 8 and 1, the sum of 2+4=6. The count of factors is 2 and 6/2=3, a prime.
MATHEMATICA
aQ[n_] := CompositeQ[n] && PrimeQ[(DivisorSigma[1, n] - n - 1)/(DivisorSigma[0, n] - 2)]; Select[Range[865], aQ] (* Amiram Eldar, Sep 07 2019 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Enoch Haga, Jan 09 2004
STATUS
approved