experimental-projects/oeis-research/Altug Alkan/Least k such that sigma(k) is a Fibonacci number when k is the product of n distinct primes/prog.sf

#!/usr/bin/ruby

# a(n) is the least k such that sigma(k) is a Fibonacci number when k is the product of n distinct primes, or 0 if no such k exists.
# https://oeis.org/A290936

# Known terms:
#   2, 94, 66, 19290, 2000006490, 247917529768610, 276320525457530886869600795810

# a(7) > 1709943212167773357407100
# a(7) = 276320525457530886869600795810

# Lower-bounds:
#   sigma(a(8)) >= fibonacci(240)

func a(n, from=1) {
    for k in (from..Inf) {

        say "[#{n}] Checking k = #{k}"
        var arr = k.fib.inverse_sigma

        with (arr.first_by { .is_squarefree_almost_prime(n) }) {|v|
            return v
        }
    }
}

var n = 8
var from = 240      # requires more than 6GB of RAM

say "a(#{n}) = #{a(n, from)}"

#~ for n in (8) {
    #~ var v = a(n)
    #~ say "#{n} #{v}"
#~ }

__END__
1 2
2 94
3 66
4 19290
5 2000006490
6 247917529768610
7 276320525457530886869600795810

[7, 276320525457530886869600795810]
[7, 277036896340045639450458794690]
[7, 278062077795208406914509455810]
[7, 278069492041326495002531471810]
[7, 283788647210397460193342594210]
[7, 284516793112050600413768772290]
[7, 285577268526665078125073654210]