- #1
PatsyTy
- 30
- 1
Hello everyone,
I'm working through some homework for a second year mathematical physics course. For the most part I am understanding everything however there is one step I do not understand regarding the steps taken to solve for the coefficients of the Legendre series. Starting with setting the function I want to expand to
\begin{equation*}
f(x)=\sum_{l=0}^\infty c_l P_l(x)
\end{equation*}
multiplying both sides by ##P_m(x)## and integrating from ##-1## to ##1## with respects to ##x## gives me
\begin{equation*}
\int_{-1}^1 f(x) P_m(x) dx = \sum_{l=0}^\infty c_l \int_{-1}^1{P_l(x) P_m (x)dx} = \sum_{l=0}^\infty c_l \frac{2}{2l+1}\delta_{lm}
\end{equation*}
The issue I am having is the next step where we go from
\begin{equation*}
\sum_{l=0}^\infty c_l \frac{2}{2l+1}\delta_{lm} \quad \text{to} \quad c_m \frac{2}{2m+1}
\end{equation*}
I looked up Kronecker Delta on wikipedia and found the property
\begin{equation*}
\sum_j \delta_{ij} a_j = a_i
\end{equation*}
however I do not understand it. I know I could just memorize this and apply it however I would like to understand where this comes from. I've tried looking up proofs of the properties of Kronecker Delta however I haven't had any luck finding them and my textbook only has a paragraph on the function. Does anyone here know of a good resource for reading up on the Kronecker Delta function?
I'm working through some homework for a second year mathematical physics course. For the most part I am understanding everything however there is one step I do not understand regarding the steps taken to solve for the coefficients of the Legendre series. Starting with setting the function I want to expand to
\begin{equation*}
f(x)=\sum_{l=0}^\infty c_l P_l(x)
\end{equation*}
multiplying both sides by ##P_m(x)## and integrating from ##-1## to ##1## with respects to ##x## gives me
\begin{equation*}
\int_{-1}^1 f(x) P_m(x) dx = \sum_{l=0}^\infty c_l \int_{-1}^1{P_l(x) P_m (x)dx} = \sum_{l=0}^\infty c_l \frac{2}{2l+1}\delta_{lm}
\end{equation*}
The issue I am having is the next step where we go from
\begin{equation*}
\sum_{l=0}^\infty c_l \frac{2}{2l+1}\delta_{lm} \quad \text{to} \quad c_m \frac{2}{2m+1}
\end{equation*}
I looked up Kronecker Delta on wikipedia and found the property
\begin{equation*}
\sum_j \delta_{ij} a_j = a_i
\end{equation*}
however I do not understand it. I know I could just memorize this and apply it however I would like to understand where this comes from. I've tried looking up proofs of the properties of Kronecker Delta however I haven't had any luck finding them and my textbook only has a paragraph on the function. Does anyone here know of a good resource for reading up on the Kronecker Delta function?