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In mathematics, the ratio test is a test (or "criterion") for the convergence of a series =, where each term is a real or complex number and a n is nonzero when n is large. The test was first published by Jean le Rond d'Alembert and is sometimes known as d'Alembert's ratio test or as the Cauchy ratio test.
If r > 1, then the series diverges. If r = 1, the root test is inconclusive, and the series may converge or diverge. The root test is stronger than the ratio test: whenever the ratio test determines the convergence or divergence of an infinite series, the root test does too, but not conversely. [1]
If r = 1, the root test is inconclusive, and the series may converge or diverge. The ratio test and the root test are both based on comparison with a geometric series, and as such they work in similar situations. In fact, if the ratio test works (meaning that the limit exists and is not equal to 1) then so does the root test; the converse ...
The convergence of a geometric series can be described depending on the value of a common ratio, see § Convergence of the series and its proof. Grandi's series is an example of a divergent series that can be expressed as 1 − 1 + 1 − 1 + ⋯ {\displaystyle 1-1+1-1+\cdots } , where the initial term is 1 {\displaystyle 1} and the common ratio ...
For a power series f defined as: = = (),where a is a complex constant, the center of the disk of convergence,; c n is the n-th complex coefficient, and; z is a complex variable.; The radius of convergence r is a nonnegative real number or such that the series converges if
First is the ratio test: [55] [56] [57] ... One important example of a test for conditional convergence is the alternating series test or Leibniz test: [62] ...
The usual argument to compute the sum of the binomial series goes as follows. Differentiating term-wise the binomial series within the disk of convergence | x | < 1 and using formula , one has that the sum of the series is an analytic function solving the ordinary differential equation (1 + x)u′(x) − αu(x) = 0 with initial condition u(0) = 1.
The ratio test can be used here: ... Graphic examples of uniform convergence of Fourier series from the University of Colorado This page was last edited on 21 ...