How do you represent the digits of a number using algebra?

Answer 1

If you mean the number of digits, this is roughly equivalent to the logarithm (base 10) of the number. However, it's a bit more tricky to find an exact expression. Rounding the logarithm up helps, but you will still be off by one, for exact powers of 10.

Answer 2

If you have a number with digits ABCD, you could represent it by 1000*A + 100*B + 10*C + D.