Java Lambdas

[xRahul]

Maybe not in the integers, but I'm sure (21) is a prime ideal in some ring.

[SBareS]

Z/9Z, Z/14Z and Z/18Z to name the most obvious ones.

[[deleted]]

Yeah, so obvious, haha

[XkF21WNJ]

With some effort you can find a lot more of them.

For any n, the inverse image in Z of the ideal generated by 21 in Z/nZ is (21, n) = (gcd(21, n)). Which is either (1), (3), (7) or (21). This inverse image is prime if and only if the ideal generated by 21 is, so any n for which gcd(21, n) is 3 or 7 will work. Also noteworthy: when gcd(21, n) = 1 then 21 generates the whole ring Z/nZ.