March 2017 Archives

On A Calculus Of Differences

The interest of my fellow students and I has been somewhat piqued of late by a curious similarity of the relationship between sequences and series to that between the derivatives and integrals of functions. Specifically, for a function f taking a non-negative argument x, we have
  F(x) = f(x) dx
  f(x) = d F(x)

and for a sequence s having terms

  s1, s2, s3, ...

we can define a series S with terms
  Sn = s1 + s2 + s3 + ... + sn = Σ si
  i = 1
where Σ is the summation sign, from which we can recover the terms of the sequence with

  sn = Sn - Sn-1

using the convention that S0 equals zero.
This similarity rather set us to wondering whether we could employ the language of calculus to reason about sequences and series.

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