## Thursday, October 29, 2015

### [U_20151029ITCX] Roger Cotes’ Integral of the Reciprocal of x^n – 1

Question

Introduction
The book VisualComplex Analysis by Tristan Needham recounts the story of RogerCotes who considered the above problem.  Without ostensibly using complex numbers, Cotes discovered a geometrical principle that helped to factorise the denominator  xn – 1,  and hence decompose the above integral.
In this article, I am going to “cheat” by using complex numbers to split up the denominator.  The fact that the denominator splits completely into a product of simple linear factors makes it easy to decompose the integrand into partial fractions.  Once this is done, I can single out the one or two fractions with purely real linear denominators.  Then I can pair up the conjugate fractions to get fractions with real quadratic denominators.  In other words, I apply a divide-and-conquer strategy, splitting up a big problem into smaller problems (Heuristics!).  Then I collect all the partial answers together to form my final answer.

Solution

Remarks
Note that I have only used real integration, not complex integration.  Complex numbers are used only to derive the various algebraic fractions.

H02. Use a diagram / model   (mentally: imagine roots of unity in a circle)
H04. Look for pattern(s)
H10. Simplify the problem
H11. Solve part of the problem
H13* Use Equation / write a Mathematical Sentence

Suitable Levels
University / college level calculus
high school students very strong and interested in calculus and complex numbers
* anybody who loves a challenging calculus problem and complex numbers