blum?: function [n][
    if any? @[n < 21, even? n, zero? n % 5]
        -> return false

    f: factors.prime n
    if or? [2 <> size f][f\0 = f\1]
        -> return false

    every? f => [3 = & % 4]
]

print "First 50 Blum numbers:"
1..∞ | select.first: 50 => blum?
     | split.every: 10
     | loop => [print map & 'x -> pad to :string x 4]

print ["\n26828th Blum number:" select.n:26828 1..∞ => blum?]