OFFSET
1,2
COMMENTS
Terms that also Fibonacci numbers are 1, 3, 5, 21, and no more below Fibonacci(300).
EXAMPLE
3 is a term since the harmonic mean of its divisors is 3/2 = Fibonacci(4)/Fibonacci(3).
15 is a term since the harmonic mean of its divisors is 5/2 = Fibonacci(5)/Fibonacci(3).
MATHEMATICA
fibQ[n_] := Or @@ IntegerQ /@ Sqrt[{5 n^2 - 4, 5 n^2 + 4}]; h[n_] := DivisorSigma[0, n]/DivisorSigma[-1, n]; q[n_] := fibQ[Numerator[(hn = h[n])]] && fibQ[Denominator[hn]]; Select[Range[1000], q]
PROG
(Python)
from itertools import islice
from sympy import integer_nthroot, gcd, divisor_sigma
def A348658(): # generator of terms
k = 1
while True:
a, b = divisor_sigma(k), divisor_sigma(k, 0)*k
c = gcd(a, b)
n1, n2 = 5*(a//c)**2-4, 5*(b//c)**2-4
if (integer_nthroot(n1, 2)[1] or integer_nthroot(n1+8, 2)[1]) and (integer_nthroot(n2, 2)[1] or integer_nthroot(n2+8, 2)[1]):
yield k
k += 1
CROSSREFS
KEYWORD
nonn
AUTHOR
Amiram Eldar, Oct 28 2021
STATUS
approved