# HornerForm of polynomials in terms of E^(i x)

I want to know how to get the HornerForm of the following expression in terms of E^(I x):

 E^(I x) + 2 E^(2 I x) + 3 E^(3 I x) + 4 E^(4 I x) + 5 E^(5 I x) +
6 E^(6 I x) + 7 E^(7 I x) + 8 E^(8 I x) + 9 E^(9 I x) + 10 E^(10 I x)


HornerForm[ expr, E^(I x)] doesn't work as well as something like Collect[ expr, E^(I x]].
How can I get the desired form?

The most straightforward way appears to be using carefully simple replacement rules involving RuleDelayed rather than Rule:

HornerForm[ E^(I x) + 2 E^(2 I x) + 3 E^(3 I x) + 4 E^(4 I x) + 5 E^(5 I x)
+ 6 E^(6 I x) + 7 E^(7 I x) + 8 E^(8 I x) + 9 E^(9 I x) + 10 E^(10 I x) /.
E^(Complex[0, b_] x) :> z^b, z] /. z :> E^(I x) % // TraditionalForm We should remember one subtlety using patterns in replacement rules involving complex (imaginary) factors, which we can illustrate with e.g.:

FullForm @ Unevaluated[ 5 I x]
FullForm[ 5 I x]

 Unevaluated[ Times[5, I, x]]
Times[ Complex[0, 5], x]


namely: built-in rewriting rules of the system automatically evaluate Times[ 5, I] to Complex[0, 5] being an atom:

AtomQ[ Complex[0, 5]]

True


therefore we couldn't make our rule simpler and had used E^(Complex[0, b_] x) :> z^b instead of something like apparently simpler E^(I b_ x) :> z^b.

• Oops apparently we took almost the same approach! :P – Leo Fang Oct 10 '13 at 1:10
• @Artes Great! Rule Replace, I got that – matheorem Oct 10 '13 at 1:16
• @Artes Great! But it seems that  Times[a_, Power[E, Times[Complex[0, b_], x]]] :> a z^b  is rebundant, right? – matheorem Oct 10 '13 at 1:22
• @matheorem yes, that was redundant, just improved. – Artes Oct 10 '13 at 1:26
• @LeoFang You had to map Power[E, Times[Complex[0, #], x]] -> y^# & because you had used Rule (->) instead of RuleDelayed (:>) – Artes Oct 10 '13 at 1:34

This only works for the special case given in your post:

input =  E^(I x) + 2 E^(2 I x) + 3 E^(3 I x) + 4 E^(4 I x) + 5 E^(5 I x) +
6 E^(6 I x) + 7 E^(7 I x) + 8 E^(8 I x) + 9 E^(9 I x) + 10 E^(10 I x);
HornerForm[input/. (Power[E, Times[Complex[0, #], x]] -> y^# & /@
Range[1, 10])] /. y -> Exp[I x]