Another way to see that some numbers are uncomputable is by pairing them off against programs. Basically, there are more numbers than programs. So there are some numbers that can't be produced by any program, i.e. can't be computed.

To see this, number all programs, as in the previous section. Then say that the th program computes a real number . Now, make a number whose first digit is different from the first digit of the number , i.e. whose first digit is different from that output by the first program. Similarly, let its second digit be different from the second digit of , i.e. from the second digit output by the second program. No, since 's first digit is different from that output by , is not the number that produces. Similarly, it isn't the number that produces. And so on. So is a number that can't be produced by any of our programs.

Wed Feb 14 23:47:23 GMT 1996