local function is_prime(n)
    assert(n % 1 == 0, "Argument must be an integer.")
    if n < 2 then return false end
    if n % 2 == 0 then return n == 2 end
    if n % 3 == 0 then return n == 3 end
    local d = 5
    while d * d <= n do
        if n % d == 0 then return false end
        d += 2
        if n % d == 0 then return false end
        d += 4
    end
    return true
end

local count = 0
local i = 42
while count < 42 do
    if is_prime(i) then
        ++count
        local str1 = string.format("%2d", count)
        local str2 = string.format("%18s", string.formatint(i))
        print($"{str1}: {str2}")
        i = 2 * i - 1
    end
    ++i
end