Problem with symbolic variables in dirac functions

Hello,

Can anybody tell me whats wrong with this:

clear all;
syms x1 x2 x0 x 'positive';
syms L 'positive';
d1=dirac(x-x0)
d2=subs(d1,{x1,x2,x0},{0,L,L/2})

The answer to this should be: d1=dirac(x-x0) d2=dirac(x-L/2)

but the response I get from Matlab:

d1 = 
dirac(x - x0)

d2 = 
dirac(L/2 - x)

why are the arguments of the dirac function in d2 (L/2-x) instead of (x-x0)? In symbolic calculations is it possible to specify somewhere that x1<x<x2 ??

Abhishek

Answer 1

[Walter Roberson]

Note: you should not be quoting anything in the syms command.

syms x1 x2 x0 x positive;

As for the dirac question: see http://www.mathworks.com/help/toolbox/symbolic/dirac.html

The Dirac delta function, dirac, has the value 0 for all x not equal to 0 and the value Inf for x = 0.

You can see from this description that the only important information is whether the value is 0 or not. Therefore the sign of the expression is irrelevant. dirac(x) is the same as dirac(-x)