OFFSET
1,3
COMMENTS
Prime divisors counted without multiplicity.
LINKS
Robert Israel, Table of n, a(n) for n = 1..2346
FORMULA
G.f. for fractions: (1/(1 - x)) * Sum_{k>=1} x^prime(k) / (prime(k)*(1 - x^prime(k))).
a(n) is the numerator of Sum_{k=1..pi(n)} floor(n/prime(k)) / prime(k).
EXAMPLE
0, 1/2, 5/6, 4/3, 23/15, 71/30, 527/210, 316/105, 117/35, 283/70, 3183/770, 5737/1155, 75736/15015, ...
MAPLE
N:= 100: # for a(1) .. a(N)
P:= select(isprime, [$1..N]):
f:= proc(n) local k;
numer(add(floor(n/P[k])/P[k], k=1..numtheory:-pi(n)))
end proc:
map(f, [$1..N]); # Robert Israel, Jan 26 2025
MATHEMATICA
Table[DivisorSum[n, 1/# &, PrimeQ[#] &], {n, 1, 33}] // Accumulate // Numerator
Table[Sum[Floor[n/Prime[k]]/Prime[k], {k, 1, n}], {n, 1, 33}] // Numerator
nmax = 33; CoefficientList[Series[1/(1 - x) Sum[x^Prime[k]/(Prime[k] (1 - x^Prime[k])), {k, 1, nmax}], {x, 0, nmax}], x] // Numerator // Rest
PROG
(PARI) a(n) = my(vp=primes(primepi(n))); numerator(sum(k=1, #vp, (n\vp[k])/vp[k])); \\ Michel Marcus, Jan 26 2025
CROSSREFS
KEYWORD
nonn,frac
AUTHOR
Ilya Gutkovskiy, Jan 20 2025
STATUS
approved