No, that's not true. The input variable of a sequence is called the index, which is a counter, such as 1, 2, 3, 4, ... Note that all of these counters are positive integers. Normallly I use index i = 1 to denote the first term of a sequence, i = 2 the second term, and so on.
Decimals and irrational numbers are completely unsuitable for serving as such counters.