If "d" is a square-free integer (a product of distinct primes) other than 1, then Q() is a quadratic extension of Q. If "d" < 0, then the class number of the ring "R" of algebraic integers of Q() is equal to 1 for precisely the following values of "d": "d" = −1, −2, −3, −7, −11, −19, −43, −67, and −163. This result was first conjectured by Gauss and proven by Kurt Heegner, although Heegner's proof was not believed until Harold Stark gave a later proof in 1967. (See Stark-Heegner theorem.) This is a special case of the famous class number problem.