-- code for https://math.stackexchange.com/questions/2805326/is-ab-a-unit-if-a-b-a-b-are-zero-divisors

function ZD(a,n)
    for b=1,n-1 do
        if (a*b)%n==0 then return true end
    end
    return false
end

function test(n)
    for a=1,n-1 do
    for b=a,n-1 do
        if ZD(a,n) and ZD(b,n) and ZD(a-b,n) and not ZD(a+b,n) then
            print(n,a,b)
            --return
        end
    end
    end
end

for n=2,1000 do
    --test(n)
end

test(42)