The sum of an infinite geometric progression converges if and only if the common ratio is less than 1 by the modulus. In your case it means |2n -1| < 1, or, equivalently,< . The solution to the last inequality are all those "n" that are remoted less than from , i.e. 0 < n < 1. Or, in the interval notation, those "n" that belong to the interval (0,1) (the ends are not included !).