OFFSET
1,2
LINKS
Chai Wah Wu, Table of n, a(n) for n = 1..10000
FORMULA
log n << a(n) << n log log n. - Charles R Greathouse IV, Aug 12 2016
EXAMPLE
a(5) = 12 because Fibonacci(12) = 144 is divisible by sigma(5) = 6.
MATHEMATICA
Table[k = 1; While[! Divisible[Fibonacci@k, DivisorSigma[1, n]], k++]; k, {n, 120}] (* Michael De Vlieger, Aug 11 2016 *)
PROG
(PARI) a(n)=my(k=1); while(fibonacci(k) % sigma(n), k++); k;
(PARI) a(n)=my(s=sigma(n), a=Mod(1, s), b=a, k=1); while(a, [a, b]=[b, a+b]; k++); k \\ Charles R Greathouse IV, Aug 12 2016
(Perl) use ntheory ":all"; sub a275789 { my($sigma, $k)=(divisor_sum(shift), 1); return 1 if $sigma==1; $k++ while (lucas_sequence($sigma, 1, -1, $k))[0]; $k; } # Dana Jacobsen, Aug 15 2016
CROSSREFS
KEYWORD
nonn
AUTHOR
Altug Alkan, Aug 09 2016
STATUS
approved