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Saturday, 24 October 2020

A subsequence converging to an analytic curve

This is a question that confronted me while I was trying to find a proof of a different result. Consider a sequence x_n \in \mathbb{R}^m with x_n \to 0. Under what circumstances does a subsequence converge to an analytic curve \gamma:[0,\varepsilon) \to \mathbb{R}^m with \gamma(0)=0? Let me make this notion precise: the sequence must converge according to all derivatives, that is, faster than any power of r = \|x\|. It should be noted that the eventual subsequence we extract need not lie on the curve, because of the existence of nonanalytic functions which converge more quickly than any polynomial. By the compactness of S^{m-1} we can pass to a subsequence such that s_1^n = \|x_n\| is converging to some point s_1 \in S^{m-1}. So we begin to construct a curve by starting with \gamma(t) = tv_1, where v_1=s_1. Next, let s^2_n be the intersection of \gamma(t) = v_1t +v_2^nt^2 with S^{n-1}, where the vector v_2^n is chosen such that \gamma(t) contains x_n. Passing to a subsequence, we have s^2_n \to s_2. We then have the curve \gamma(t) = v_1t +v_2t^2, where v_2 is chosen so that the curve intersects S^{m-1} at s_2. We assume for the moment that such a v_2 exists, and examine this assumption shortly. Note that the distance from x_n to the curve is bounded by ct_n^2, where \gamma(t_n) is the point of the curve closest to x_n, and since s^2_n \to s_2, c can be made as small as we like by truncating the sequence. Iterating this process, we arrive at a curve \gamma(t) = v_1t +v_2t^2 + \ldots + v_kt^k, and a subsequence x_n such that d(x_n,\gamma(t_n)) \leq ct_n^k \leq cr_n^k, where r_n =||x_n||. Now we return to the question of whether the vector v_2 (and v_3,\ldots,v_k) actually exists. It can happen that as s^2_n \to s_2, v_2^n becomes unbounded and consequently v_2 doesn't exist. This means that the sequence is converging to v_1 slower than t^2. This corresponds to the case where the curve must be written as a Puiseux series, rather than a Taylor series. In this case, we multiply the Taylor series by t, i.e. we consider \gamma(t) = v_1t^2 +v_2t^3 + \ldots + v_kt^{k+1}, and again try to construct v_2. It could happen that v_2 = 0, in which case we attempt to construct v_3, continuing to multiply by t whenever a vector fails to exist. Eventually we will obtain the first two non-zero terms of our curve \gamma(t) = v_1t^{1+l} + v_jt^{j+l} for some j \geq 2. Otherwise, all subsequences of the sequence x_n must be asymptoting to v_1 slower than any rational power \rho of r, \rho > 1, \rho \to 1. From here, we can construct all remaining terms using our original process, i.e. all remaining vectors v_i will exist. Thus, after multiplying by t enough times, we will eventually be able to construct a curve \gamma(t) = v_1t^{1+l} +v_2t^{2+l} + \ldots + v_kt^{k}, which satisfies our requirements. However, it's unfortunately possible for x_n to be asymptoting to v_1 slower than any rational power or r. But, it means it's converging to zero so fast relative to its rate of rotation that I believe it is possible to find a higher dimensional manifold that it is asymptoting to. I'm working on making this notion precise in a paper I'm working on.