is_practical_number.sf

#!/usr/bin/ruby

# Efficient algorithm, due to M. F. Hasler (Jan 13 2013), for efficiently
# checking if a given number is a practical number, using its prime factorization.

# See also:
#   https://oeis.org/A005153
#   https://rosettacode.org/wiki/Practical_numbers

func is_practical(n) {

    n.is_odd && return (n == 1)
    n.is_pos || return false

    var p = 1
    var f = n.factor_exp

    f.each_cons(2, {|a,b|
        p *= sigma(a.head**a.tail)
        b.head > (1 + p) && return false
    })

    return true
}

say 20.by { is_practical(_) }   #=> [1, 2, 4, 6, 8, 12, 16, 18, 20, 24, 28, 30, 32, 36, 40, 42, 48, 54, 56, 60]

say is_practical(666)     #=> true
say is_practical(6666)    #=> true
say is_practical(66666)   #=> false