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The Conway Series

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Describe the sequence:

a(1)=a(2)=1, a(n) = a(a(n-1)) + a(n-a(n-1)) for n>2.


This sequence and its remarkable properties were discovered, apparently independently, by Douglas Hofstadter (circa 1975), John Conway (in the early 1980's), B.W. Connoly, and others. Since Conway discovered many of the deeper properties, and is the one responsible for popularizing the sequence, it seems appropriate to name the sequence after him.

Some properties of a(n):

In a lecture at Bell Labs in 1988, Conway asked for the rate at which a(n)/n converges to 1/2. In particular, he asked for N(epsilon), the last value of n such that |a(n)/n - 1/2| exceeds epsilon, in the case epsilon=1/20. This problem was solved by Colin Mallows of Bell Labs: he found that N(1/20)=1489 and N(1/40)=6083008742. These values are reported in his article in the American Mathematical Monthly, January 1991, p. 5.

Conway's sequence has a deep combinatorial structure, and is intimately connected with all of the following: variants of Pascal's triangle, the Gaussian distribution, combinatorial operations on finite sets, coin-pushing games, and Fibonacci and Catalan numbers.