matlab-use-symbolic-math

# MATLAB Symbolic Math Toolbox

This skill provides guidelines, correct syntax, and common patterns for generating MATLAB® code that uses Symbolic Math Toolbox.

## When to Use This Skill

- Creating or manipulating symbolic variables, expressions, and functions
- Performing symbolic differentiation, integration, limits, or summation
- Simplifying, factoring, expanding, or collecting symbolic expressions
- Computing Laplace, Fourier, or Z-transforms and their inverses
- Deriving transfer functions or state-space equations from differential equations
- Displaying or plotting symbolic expressions
- Using variable precision arithmetic (VPA)
- Generating MATLAB functions, Simulink function blocks, Simscape equations, and C code from symbolic expressions
- Extracting PDE coefficients for use with PDE Toolbox
- Converting symbolic expressions to C code or standalone executables
- Matrix-level (atomic, textbook-style) symbolic linear algebra with `symmatrix`
- Using physical units or constants in symbolic computations with `symunit`
- Rewriting or combining algebraic expressions into specific forms

## When NOT to Use This Skill

- Purely numeric computation with no symbolic variables (use standard MATLAB numeric functions)
- Statistics, machine learning, or data analysis on numeric datasets
- Image processing, signal processing, or other toolbox-specific workflows that don't involve symbolic math
- String manipulation or file I/O operations
- When the user explicitly asks for numeric approximations only (use `double` or numeric solvers directly)
- PDE Toolbox mesh generation, boundary conditions, or solving (downstream of coefficient extraction)
- Numeric linear algebra (use standard MATLAB matrix operations)

## Critical Rules

### 1. NEVER Pass Strings or Character Vectors to Symbolic Functions

**WRONG (deprecated — warns today, errors in a future release; the single `=` in `solve` errors now):**
```matlab
solve('x^2 + 2*x - 3 = 0')
dsolve('Dy = -a*y')
```

**CORRECT:**
```matlab
syms x
solve(x^2 + 2*x - 3 == 0, x)

syms y(t) a
dsolve(diff(y,t) == -a*y)
```

### 2. Use `syms` for Interactive Work, `sym` for Functions and Constants

- **`syms x y z`** — Creates fresh symbolic variables and clears any prior assumptions. Use for interactive scripts and Live Scripts.
- **`x = sym('x')`** — Refers to a symbolic variable. Inherits existing assumptions. Required inside MATLAB functions (not scripts) because `syms` dynamically creates workspace variables.
- **`sym(pi)`** — Converts numeric to exact symbolic. Use for symbolic constants.
- **`sym('pi')`** — Creates a symbolic *variable named* `pi`, NOT the mathematical constant π. This is a common source of confusion.

**WRONG:**
```matlab
% Inside a function:
function result = myFunc()
    syms x          % Error or unreliable in compiled/nested functions
    result = x^2;
end

% Creating symbolic constant pi:
p = sym('pi');      % Creates variable named "pi", NOT the constant
```

**CORRECT:**
```matlab
% Inside a function:
function result = myFunc()
    x = sym('x');   % Use sym inside functions
    result = x^2;
end

% Creating symbolic constant pi:
p = sym(pi);        % Converts numeric pi to exact symbolic π
```

### 3. Assumption Management

Assumptions persist in the symbolic engine even after `clear`. This is a frequent source of subtle bugs.

```matlab
% Setting assumptions
syms x real                  % x is real (clears prior assumptions)
syms n positive integer      % n is a positive integer
assume(x > 0)                % x is positive (REPLACES all prior assumptions on x)
assumeAlso(x < 10)           % ADDS assumption: 0 < x < 10

% Checking assumptions
assumptions(x)               % Shows assumptions on x
assumptions                  % Shows ALL assumptions in workspace

% Clearing assumptions — two correct ways:
syms x                       % Recreate with syms: clears assumptions
assume(x, 'clear')           % Explicitly clear assumptions on x
reset(symengine)             % Nuclear option: clears EVERYTHING
```

**Best Practice:** Use `syms x` to clear assumptions (it resets the variable fresh). Use `assume(x, 'clear')` when you need to reset a specific variable mid-script. The MATLAB `clear` command only removes workspace variables — it has no effect on the symbolic engine's assumption store.

### 4. `subs` Does Not Modify In-Place

The `subs` function returns a new expression. It does NOT modify the original.

**WRONG:**
```matlab
syms x
f = x^2 + 3*x;
subs(f, x, 2);         % Result is discarded!
disp(f)                % Still x^2 + 3*x
```

**CORRECT:**
```matlab
syms x
f = x^2 + 3*x;
f_val = subs(f, x, 2);    % Assign the result
% or: f = subs(f, x, 2);  % Overwrite f
```

### 5. Do Not Wrap Numeric Literals in `sym()` Inside Symbolic Expressions

AI tools frequently over-wrap every numeric literal in `sym()`.
When any operand in an arithmetic expression is symbolic, MATLAB automatically promotes all numeric literals in that expression to symbolic. Wrapping literals in `sym()` adds clutter and can cause errors.
**When you DO need `sym()`:** Only when creating a standalone symbolic number with NO symbolic variables present in the expression.

```matlab
% No symbolic variable involved — sym() IS needed:
half = sym(1/2);                % Exact 1/2, not 0.5 double
half = sym(1)/2;                % Exact 1/2, declaring sym(1) promotes all numeric literals to symbolic
piExact = sym(pi);              % Exact π, not 3.14159...

% Symbolic variable already present — sym() is NOT needed:
syms x
f = x/2 + 1/3;                 % Automatically exact: x/2 + 1/3
g = exp(-x^2/2) / sqrt(2*pi);  % All literals promoted by x
```

## Core Workflow Patterns

### Creating Variables and Expressions

```matlab
% Multiple variables at once
syms a b c

% Variables with assumptions
syms a b c real
syms n positive integer
syms x
assume(x > 2)


% Symbolic matrices with auto-generated elements
syms A [3 3]                 % Creates A = [A1_1 A1_2 A1_3; ...