# Integrating hat functions

I am trying to integrate a hat function for a project that I am doing and have found a method to do so but I find it sloppy. Currently I have the basis function

\[Psi][z_] := z - Subscript[Z, i]/ \[CapitalDelta]z + 1;


which I am trying to integrate from $z_{i-1}$ to $z_{i+1}$. I break the basis function up into two pieces and integrate the left side from $z_{i-1}$ to $z_{i}$ and then the right side from $z_i$ to $z_{i+1}$. My first question is, is there a way to integrate piecewise functions? The second question I have is, is there a way to set global assumptions like $z_{i-1} < z_i < z_{i+1}$, $z_i - z_{i-1} = \Delta z$ , etc?

Edit: This is the piece wise function taken directly from my code I am trying to integrate

\[Psi][z_, c_] :=  Piecewise[{{(z - c)/\[CapitalDelta]z + 1,
z <= c}, {-(z - c)/\[CapitalDelta]z + 1, z > c}}];


where $c$ is the center of the hat function. Here is my attempt to integrate the piece wise function

 FullSimplify[
Integrate[\[Psi][z, Subscript[Z, i]], {z, Subscript[Z, i - 1],
Subscript[Z, i + 1]}],
Assumptions -> {-(Subscript[Z, i + 1] - Subscript[Z,
i ]) == -\[CapitalDelta]z,
Subscript[Z, i + 1] - Subscript[Z,
i ] == \[CapitalDelta]z, -(Subscript[Z, i] - Subscript[Z,
i - 1 ]) == -\[CapitalDelta]z,
Subscript[Z, i] - Subscript[Z, i - 1 ] == \[CapitalDelta]z}]


I do not get a usable answer. Am I doing something wrong (ie can one integrate a piece wise function)?

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Why are using using global variables not good? I would never know the value of $Z_i$ in the scope of this project and $\Delta z$ can have many values so I don't really see how else I could express the function. – rioneye Feb 13 '13 at 4:58
Have you looked at Piecewise? Did you know that Integrate can take symbolic values (such as Subscript[z,i-1]) in its integration limits? – Xerxes Feb 13 '13 at 5:46
@Nasser: Thanks for the tip! However, the value of $Z_i$ is some undeclared constant in my code, as are $Z_{i+1}$, $Z_{i-1}$. – rioneye Feb 13 '13 at 22:38
@Xerxes: Yes I have. See the update. – rioneye Feb 13 '13 at 22:38

Short answer: Mathematica has no problem integrating piecewise or hat functions.

Your notation seems to me to be needlessly complex. Why bother to define $Z_i$ when it's just $Z_0+i\Delta z$? Isn't your $\psi$ just 1-Abs[z-c]/Δz? However, I'll try to adhere to the spirit of your notation. Here's some code that works for me:

Zdefs = {Subscript[Z, i] - Subscript[Z, i - 1] == Δz,
Subscript[Z, i + 1] - Subscript[Z, i] == Δz};
Integrate[ψ[z, Subscript[Z, i]],
{z, Subscript[Z, i - 1], Subscript[Z, i + 1]} /.
Solve[Zdefs, {Subscript[Z, i + 1], Subscript[Z, i - 1]}][[1]],
Assumptions -> Δz > 0 &&
Subscript[Z, i] ∈ Reals]

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Thanks a bunch Xerxes! I was emulating the notation used in our analytical solution. It makes the code easier to compare to our results. I do have a few questions for you though. I don't understand why you use this : /. Solve[Zdefs, {Subscript[Z, i + 1], Subscript[Z, i - 1]}][[1]] ? Also what does this [[1]] mean? Sorry if these questions are especially basic. – rioneye Feb 13 '13 at 23:32
Since your definitions of $Z_{i\pm 1}$ can be reduced to functions of $Z_i$ and $\Delta z$, Solve will do that for us. The [[1]] tells Mathematica to take the first (and only) solution to those equations. You don't have to do that; you can instead add Subscript[Z, i - 1] < Subscript[Z, i] < Subscript[Z, i + 1] to your Assumptions. But then you'll have to make the substitution later anyway. – Xerxes Feb 13 '13 at 23:39

(* New assumptions so Integrate can do its work *)
Assuming[{z \[Element] Reals,
c \[Element] Reals, \[CapitalDelta]z \[Element] Reals,
Subscript[Z, i - 1] \[Element] Reals,
Subscript[Z, i + 1] \[Element] Reals},

(* The rest is just copied from the question *)
FullSimplify[
Integrate[\[Psi][z, Subscript[Z, i]], {z, Subscript[Z, i - 1],
Subscript[Z, i + 1]}],
Assumptions -> {-(Subscript[Z, i + 1] - Subscript[Z,
i ]) == -\[CapitalDelta]z,
Subscript[Z, i + 1] - Subscript[Z,
i ] == \[CapitalDelta]z, -(Subscript[Z, i] - Subscript[Z,
i - 1 ]) == -\[CapitalDelta]z,
Subscript[Z, i] - Subscript[Z, i - 1 ] == \[CapitalDelta]z}]]


Result:

$\begin{array}{cc} \{ & \begin{array}{cc} \frac{3 \left(Z_i-Z_{i+1}\right){}^2}{\text{$\Delta $z}} & Z_i>Z_{i+1} \\ \frac{\left(Z_i-Z_{i+1}\right){}^2}{\text{$\Delta $z}} & Z_i<Z_{i+1} \\ \end{array} \\ \end{array}$

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