Primefinder

function isPrime(n)
    local n = tonumber(n)
    --catch nil, 0, 1, negative and non int numbers
    if not n or n<2 or (n % 1 ~=0) then
        return false
    --catch even number above 2
    elseif n>2 and (n % 2 == 0) then
        return false
    --primes over 5 end in 1,3,7 or 9
    --catch numbers that end in 5 or 0 (multiples of 5)
    elseif n>5 and (n % 5 ==0) then
        return false
    --now check for prime
    else
        --only do the odds
        for i = 3, math.sqrt(n), 2 do
            --did it divide evenly
            if (n % i == 0) then
                return false
            end
        end
        --can defeat optimus
        return true
    end
end

local number = 0
if fs.exists("/cpr") then
	local file = fs.open("/cpr", "r")
	number = tonumber(file.readAll())
	file.close()
end

while true do
	number = number+1

	if isPrime(number) then
		local file = fs.open("/cpr", "w")
		file.write(tostring(number))
		file.close()
	end
	os.queueEvent("")
	coroutine.yield()
end