OFFSET
1,2
COMMENTS
A000961 (power of primes) is a subsequence, since for a prime power all divisors are divisible by all smaller divisors.
The list of numbers that are not prime powers begin: 42, 54, 66, 78, ...
LINKS
Reinhard Zumkeller, Table of n, a(n) for n = 1..10000
EXAMPLE
Divisors of 27 are: 27, 9, 3, 1 and (27/9 + 27/3 + 27/1) + (9/3 + 9/1) + 3/1 = 54, an integer.
Divisors of 6 are: 6, 3, 2, 1 and (6/3 + 6/2 + 6/1) + (3/2 + 3/1) + 2/1 = 35/2 not an integer, so 6 is not here.
PROG
(PARI) isok(n) = {d = divisors(n); s = 0; forstep(k = #d-1, 1, -1, forstep(j=k-1, 1, -1, s += d[k]/d[j]; ); ); denominator(s) == 1; }
(Haskell)
import Data.List (tails); import Data.Ratio ((%), denominator)
a247199 n = a247199_list !! (n-1)
a247199_list = filter f [1..] where
f x = 1 == denominator
(sum [v % w | (v:ws) <- tails $ reverse $ a027750_row x, w <- ws])
-- Reinhard Zumkeller, Nov 25 2014
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Marcus, Nov 25 2014
STATUS
approved