with (200) {|n|
    say "Mertens function in the range 1..#{n}:"
    (1..n).map { mertens(_) }.slices(20).each {|line|
        say line.map{ "%2s" % _ }.join(' ')
    }
}

with (1000) {|n|
    say "\nIn the range 1..#{n}, there are:"
    say (1..n->count_by { mertens(_)==0 }, " zeros")
    say (1..n->count_by { mertens(_)==0 && mertens(_-1)!=0 }, " zero crossings")
}