On the Order of Power Series and the Sum of Square Roots Problem
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This paper focuses on the study of the order of power series that are linear combinations of a given finite set of power series. The order of a formal power series, known as ord(f), is defined as the minimum exponent of x that has a non-zero coefficient in f(x). Our first result is that the order of the Wronskian of these power series is equivalent up to a polynomial factor, to the maximum order which occurs in the linear combination of these power series. This implies that the Wronskian approach used in (Kayal and Saha, TOCT'2012) to upper bound the order of sum of square roots is optimal up to a polynomial blowup. We also demonstrate similar upper bounds, similar to those of (Kayal and Saha, TOCT'2012), for the order of power series in a variety of other scenarios. We also solve a special case of the inequality testing problem outlined in (Etessami et al., TOCT'2014). In the second part of the paper, we study the equality variant of the sum of square roots problem, which is decidable in polynomial time due to (Blömer, FOCS'1991). We investigate a natural generalization of this problem when the input integers are given as straight line programs. Under the assumption of the Generalized Riemann Hypothesis (GRH), we show that this problem can be reduced to the so-called “one dimensional” variant. We identify the key mathematical challenges for solving this “one dimensional” variant.
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