Lesson 11: Symbolic Mathematics

Create the symbolic variables x and y.

x = sym('x')

x = x

y = sym('y')

y = y

Create a 1×4 symbolic vector a with automatically generated elements a1, ..., a4.

a = sym('a',[1 4])

a =  [ a1, a2, a3, a4]

Format the names of elements of a by using a format character vector as the first argument. sym replaces %d in the format character vector with the index of the element to generate the element names.

a = sym('x_%d',[1 4])

a =  [ x_1, x_2, x_3, x_4]

This syntax does not create symbolic variables x_1, ..., x_4 in the MATLAB workspace. Access elements of a using standard indexing methods.

a(1)
a(2:3)

ans =  x_1
ans =  [ x_2, x_3]

Create a 3×4 symbolic matrix with automatically generated elements. The elements are of the form Ai_j, which generates the elements A1_1, ..., A3_4.

A = sym('A', [3 4])

A =
[ A1_1, A1_2, A1_3, A1_4]
[ A2_1, A2_2, A2_3, A2_4]
[ A3_1, A3_2, A3_3, A3_4]

Create a 4×4 matrix with the element names x_1_1, ..., x_4_4 by using a format character vector as the first argument. sym replaces %d in the format character vector with the index of the element to generate the element names.

B = sym('x_%d_%d',4)

B =
[ x_1_1, x_1_2, x_1_3, x_1_4]
[ x_2_1, x_2_2, x_2_3, x_2_4]
[ x_3_1, x_3_2, x_3_3, x_3_4]
[ x_4_1, x_4_2, x_4_3, x_4_4]

This syntax does not create symbolic variables A1_1, ..., A3_4, x_1_1, ..., x_4_4 in the MATLAB workspace. To access an element of a matrix, use parentheses.

A(2,3)
B(4,2)

ans =  A2_3
ans =  x_4_2

Create a 2×2×2 symbolic array with automatically generated elements a_1,1,1,…,a_2,2,2.

A = sym('a',[2 2 2])

A(:,:,1) =
[ a1_1_1, a1_2_1]
[ a2_1_1, a2_2_1]

A(:,:,2) =
[ a1_1_2, a1_2_2]
[ a2_1_2, a2_2_2]

Convert numeric values to symbolic numbers or expressions. Use sym on subexpressions instead of the entire expression for better accuracy. Using sym on entire expressions is inaccurate because MATLAB first converts the expression to a floating-point number, which loses accuracy. sym cannot always recover this lost accuracy.

inaccurate1 = sym(1/1234567)

inaccurate1 = \(\frac{{{\rm{7650239286923505}}}}{{{\rm{9444732965739290427392}}}}\)

accurate1 = 1/sym(1234567)

accurate1 = \(\frac{1}{{1234567}}\)

inaccurate2 = sym(sqrt(1234567))

inaccurate2 = \(\frac{{{\rm{4886716562018589}}}}{{{\rm{4398046511104}}}}\)

accurate2 = sqrt(sym(1234567))

accurate2 = \(\sqrt {1234567} \)

inaccurate3 = sym(exp(pi))

inaccurate3 = \(\frac{{{\rm{6513525919879993}}}}{{{\rm{281474976710656}}}}\)

accurate3 = exp(sym(pi))

accurate3 = \({e^\pi }\)

When creating symbolic numbers with 15 or more digits, use quotation marks to accurately represent the numbers.

inaccurateNum = sym(11111111111111111111)

inaccurateNum = 11111111111111110656

accurateNum = sym('11111111111111111111')

accurateNum = 11111111111111111111

When you use quotation marks to create symbolic complex numbers, specify the imaginary part of a number as 1i, 2i, and so on.

sym('1234567 + 1i')

ans = 1234567+i

Create a symbolic expression and a symbolic matrix from anonymous functions associated with MATLAB handles.

h_expr = @(x)(sin(x) + cos(x));
sym_expr = sym(h_expr)

sym_expr = cos(x)+sin(x)

h_matrix = @(x)(x*pascal(3));
sym_matrix = sym(h_matrix)

sym_matrix =
[ x, x, x]
[ x, 2*x, 3*x]
[ x, 3*x, 6*x]

Create the symbolic variables x, y, z, and t while simultaneously assuming that x is real, y is positive, z rational, and t is positive integer.

x = sym('x','real');
y = sym('y','positive');
z = sym('z','rational');
t = sym('t',{'positive','integer'});

Check the assumptions on x, y, z, and t using assumptions.

assumptions

ans =  [ in(x, 'real'), in(z, 'rational'), 1 <= t, 0 < y, in(t, 'integer')]

meaning: ans = \((x \in ℝ \quad z \in ℚ \quad 1 \le t\quad 0 < y\quad t \in ℤ )\)

For further computations, clear the assumptions using assume.

assume([x y z t],'clear')
assumptions

ans =
Empty sym: 1-by-0

Create a symbolic matrix and set assumptions on each element of that matrix.

A = sym('A%d%d',[2 2],'positive')

A = $\left( {\begin{array}{*{20}{c}}
{{A_{11}}}&{{A_{12}}}\\
{{A_{21}}}&{{A_{22}}}
\end{array}} \right)$

Solve an equation involving the first element of A. MATLAB assumes that this element is positive.

solve(A(1,1)^2-1, A(1,1))

ans = 1

Check the assumptions set on the elements of A by using assumptions.

assumptions(A)

ans = [ 0 < A11, 0 < A12, 0 < A21, 0 < A22]

Clear all previously set assumptions on elements of a symbolic matrix by using assume.

assume(A,'clear');
assumptions(A)

ans =
Empty sym: 1-by-0

Solve the same equation again.

solve(A(1,1)^2-1, A(1,1))

ans =
-1
1

Convert pi to a symbolic value.

Choose the conversion technique by specifying the optional second argument, which can be 'r', 'f', 'd', or 'e'. The default is 'r'. See the Input Arguments section for the details about conversion techniques.

r = sym(pi)

r = \(\pi \)

f = sym(pi,'f')

f = \(\frac{{{\rm{884279719003555}}}}{{{\rm{281474976710656}}}}\)

d = sym(pi,'d')

d = 3.1415926535897931159979634685442

e = sym(pi,'e')

e = \(\pi {\rm{ - }}\frac{{198 esp}}{{359}}\)

Create symbolic variables x and y.

syms x y

Create a 1×4 symbolic vector a with the automatically generated elements a1, ..., a4.

syms a [1 4]
a

a =  [ a1, a2, a3, a4]

Change the naming format of the generated elements by using a format character vector. Declare the symbolic variables by enclosing each variable name in single quotes. syms replaces %d in the format character vector with the index of the element to generate the element names.

syms 'a_%d' 'b_%d' [1 4]
a
b

a = [ a_1, a_2, a_3, a_4]
b = [ b_1, b_2, b_3, b_4]

Create symbolic variables x and equation S.

syms x
S = x^2 - 2

Create symbolic functions with one and two arguments.

syms s(t) f(x,y)

Both s and f are abstract symbolic functions. They do not have symbolic expressions assigned to them, so the bodies of these functions are s(t) and f(x,y), respectively.

Specify the following formula for f

f(x,y) = x + 2*y

Compute the function value at the point x = 1 and y = 2.

f(1,2)

ans =  5

Create a symbolic function and specify its formula by using a symbolic matrix.

syms x
M = [x x^3; x^2 x^4];
f(x) = M

f(x) =
[ x, x^3]
[ x^2, x^4]

Compute the function value at the point x = 2:

f(2)

ans =
[ 2, 8]
[ 4, 16]

Compute the value of this function for x = [1 2 3; 4 5 6]. The result is a cell array of symbolic matrices.

xVal = [1 2 3; 4 5 6];
y = f(xVal)

y =  2×2 cell array
{2×3 sym} {2×3 sym}
{2×3 sym} {2×3 sym}

Access the contents of a cell in the cell array by using braces.

y{1}

ans =
[ 1, 2, 3]
[ 4, 5, 6]

Create some symbolic variables, functions, and arrays.

syms a f(x)
syms A [2 2]

Display a list of all symbolic objects that currently exist in the MATLAB workspace by using syms.

syms

Your symbolic variables are:
A A1_1 A1_2 A2_1 A2_2 a f x

Instead of displaying a list, return a cell array of all symbolic objects by providing an output to syms.

S = syms

S =  8×1 cell array
{'A' }
{'A1_1'}
{'A1_2'}
{'A2_1'}
{'A2_2'}
{'a' }
{'f' }
{'x' }

Create several symbolic objects.

syms a b c f(x)

Return all symbolic objects as a cell array by using the syms function. Use the clear function to delete all symbolic objects in the MATLAB workspace.

clear

Check that you deleted all symbolic objects by calling syms. The output is empty, meaning no symbolic objects exist in the MATLAB workspace.

syms

If you set a variable equal to a symbolic expression and then apply the syms command to the variable, MATLAB software removes the previously defined expression from the variable. For example,

syms a b
f = a + b

returns

f = a + b

If later you enter

syms f
f

then MATLAB removes the value a + b from the expression f:

f =
f

You can use the syms command to clear variables of definitions that you previously assigned to them in your MATLAB session. syms clears the assumptions of the variables: complex, real, integer, and positive. These assumptions are stored separately from the symbolic object. However, recreating a variable using sym does not clear its assumptions. For more information, see Delete Symbolic Objects and Their Assumptions.

The syms function creates a variable dynamically. For example, the command syms x creates the symbolic variable x and automatically assigns it to a MATLAB variable with the same name.

syms x
x

x = x

The sym function refers to a symbolic variable, which you can then assign to a MATLAB variable with a different name. For example, the command f1 = sym('x') refers to the symbolic variable x and assigns it to the MATLAB variable f1.

f1 = sym('x')

f1 = x

Use the syms function to create a symbolic variable x and automatically assign it to a MATLAB variable x. When you assign a number to the MATLAB variable x, the number is represented in double-precision and this assignment overwrites the previous assignment to a symbolic variable. The class of x becomes double.

syms x
x = 1/33

x = 0.0303

class(x)

ans =  'double'

Use the sym function to refer to an exact symbolic number without floating-point approximation. You can then assign this number to the MATLAB variable x. The class of x is sym.

x = sym('1/33')

x = 1/33

class(x)

ans =  'sym'

When you create a symbolic variable with an assumption, MATLAB stores the symbolic variable and its assumption separately.

Use syms to create a symbolic variable that is assigned to a MATLAB variable with the same name. You get a fresh symbolic variable with no assumptions. If you declare a variable using syms, existing assumptions are cleared.

syms x positive
syms x
assumptions

ans =
Empty sym: 1-by-0

Use sym to refer to an existing symbolic variable. If this symbolic variable was used in your MATLAB session before, then sym refers to it and its current assumption. If it was not used before, then sym creates it with no assumptions.

syms x positive
x = sym('x');
assumptions

ans = 0 < x

To create many symbolic variables simultaneously, using the syms function is more convenient. You can create multiple variables in one line of code.

syms a b c

When you use sym, you have to declare MATLAB variables one by one and refer them to the corresponding symbolic variables.

a = sym('a');
b = sym('b');
c = sym('c');

To declare a symbolic array that contains symbolic variables as its elements, you can use either syms or sym.

The command syms a [1 3] creates a 1-by-3 symbolic array a and the symbolic variables a1, a2, and a3 in the workspace. The symbolic variables a1, a2, and a3 are automatically assigned to the symbolic array a.

clear all
syms a [1 3]
a

a =
[ a1, a2, a3]

whos

Name Size Bytes Class Attributes
a 1x3 8 sym
a1 1x1 8 sym
a2 1x1 8 sym
a3 1x1 8 sym

The command a = sym('a',[1 3]) refers to the symbolic variables a1, a2, and a3, which are assigned to the symbolic array a in the workspace. The elements a1, a2, and a3 are not created in the workspace.

clear all
a = sym('a',[1 3])

a =
[ a1, a2, a3]

whos

Name Size Bytes Class Attributes
a 1x3 8 sym

To declare a symbolic variable within a nested function, use sym. For example, you can explicitly define a MATLAB variable x in the parent function workspace and refer x to a symbolic variable with the same name.

function primaryFx
x = sym('x')
function nestedFx
...
end
end

Nested functions make the workspace static, so you cannot dynamically add variables using syms.

Suppose you want to use a symbolic variable to represent the golden ratio

\(\phi = \frac{{1 + \sqrt 5 }}{2}\)

The command

phi = (1 + sqrt(sym(5)))/2;

achieves this goal. Now you can perform various mathematical operations on phi. For example,

f = phi^2 - phi - 1

f =  (5^(1/2)/2 + 1/2)^2 - 5^(1/2)/2 - 3/2

Now suppose you want to study the quadratic function \(f = a{x^2} + bx + c\). First, create the symbolic variables a, b, c, and x:

syms a b c x

Then, assign the expression to f:

f = a*x^2 + b*x + c;

To create a symbolic number, use the sym command. Do not use the syms function to create a symbolic expression that is a constant. For example, to create the expression whose value is 5, enter f = sym(5). The command f = 5 does not define f as a symbolic expression.

Combine these symbolic inequalities into a logical condition by using |.

syms x y
xy = x>=0 | y>=0;

Set the assumption represented by the condition using assume.

assume(xy)

Verify that the assumptions are set.

assumptions

ans =
0 <= x | 0 <= y

Define a range for a variable by combining two inequalities into a logical condition using &.

syms x
range = 0 < x & x < 1;

Return the condition at 1/2 and 10 by substituting for x using subs. The subs function does not evaluate the conditions automatically.

x1 = subs(range,x,1/2)
x2 = subs(range,x,10)

x1 =
0 < 1/2 & 1/2 < 1
x2 =
0 < 10 & 10 < 1

Evaluate the inequalities to logical 1 or 0 by using isAlways.

isAlways(x1)
isAlways(x2)

ans = logical
1
ans =  logical
0

Solve this trigonometric equation. Define the equation by using the == operator.

syms x
eqn = sin(x) == cos(x);
solve(eqn, x)

ans = pi/4

Plot the equation \(\sin ({x^2}) = \sin ({y^2})\) by using fimplicit. Define the equation by using the == operator.

syms x y
eqn = sin(x^2) == sin(y^2);
fimplicit(eqn)

Test the equality of two symbolic expressions by using isAlways.

syms x
eqn = x+1 == x+1;
isAlways(eqn)

ans =  logical
1

eqn = sin(x)/cos(x) == tan(x);
isAlways(eqn)

ans =  logical
1

Check the equality of two symbolic matrices by using isAlways.

A = sym(hilb(3));
B = sym([1 1/2 5; 1/2 2 1/4; 1/3 1/8 1/5]);
isAlways(A == B)

ans =  3×3 logical array
1 1 0
1 0 1
1 0 1

Compare a matrix and a scalar. The == operator expands the scalar into a matrix of the same dimensions as the input matrix.

A = sym(hilb(3));
B = sym(1/2);
isAlways(A == B)

ans =  3×3 logical array
0 1 0
1 0 0
0 0 0

syms x
p = (x - 2)*(x - 4);
expand(p)

ans = x^2 - 6*x + 8

Expand the trigonometric expression cos(x + y). Simplify the cos function input x + y to x or y by applying standard identities.

syms x y
expand(cos(x + y))

ans =  cos(x)*cos(y) - sin(x)*sin(y)

Expand \({e^{{{(a + b)}^2}}}\). Simplify the exp function input, \({{{(a + b)}^2}}\), by applying standard identities.

syms a b
f = exp((a + b)^2);
expand(f)

ans =  exp(a^2)*exp(b^2)*exp(2*a*b)

Expand expressions in a vector. Simplify the inputs to functions in the expressions by applying identities.

syms t
V = [sin(2*t), cos(2*t)];
expand(V)

ans =  [ 2*cos(t)*sin(t), 2*cos(t)^2 - 1]

By default, expand both expands terms raised to powers and expands functions by applying identities that simplify inputs to the functions. Expand only terms raised to powers and suppress expansion of functions by using 'ArithmeticOnly'.

Expand \({\left( {\sin (3x) - 1} \right)^2}\). By default, expand will expand the power ^2 and simplify the sin input 3*x to x.

syms x
f = (sin(3*x) - 1)^2;
expand(f)

ans =  2*sin(x) + sin(x)^2 - 8*cos(x)^2*sin(x) - 8*cos(x)^2*sin(x)^2 + 16*cos(x)^4*sin(x)^2 + 1

Suppress expansion of functions, such as sin(3*x), by setting ArithmeticOnly to true.

expand(f, 'ArithmeticOnly', true)

ans =  sin(3*x)^2 - 2*sin(3*x) + 1

Simplify the input of log function calls. By default, expand does not simplify logarithm input because the identities used are not valid for complex values of variables.

syms a b c
f = log((a*b/c)^2);
expand(f)

ans =  log((a^2*b^2)/c^2)

Apply identities to simplify the input of logarithms by setting 'IgnoreAnalyticConstraints' to true.

expand(f,'IgnoreAnalyticConstraints',true)

ans =  2*log(a) + 2*log(b) - 2*log(c)

F = factor(823429252)

F =  2 2 59 283 12329

To factor integers greater than flintmax, convert the integer to a symbolic object using sym. Then place the number in quotation marks to represent it accurately.

F = factor(sym('82342925225632328'))

F =  [ 2, 2, 2, 251, 401, 18311, 5584781]

To factor a negative integer, convert it to a symbolic object using sym.

F = factor(sym(-92465))

F =  [ -1, 5, 18493]

Factor the fraction 112/81 by converting it into a symbolic object using sym.

F = factor(sym(112/81))

F =  [ 2, 2, 2, 2, 7, 1/3, 1/3, 1/3, 1/3]

Factor the polynomial \({x^6} - 1 \).

syms x
F = factor(x^6-1)

F =  [ x - 1, x + 1, x^2 + x + 1, x^2 - x + 1]

Factor the polynomial \({y^6} - {x^6}\).

syms y
F = factor(y^6-x^6)

F =  [ -1, x - y, x + y, x^2 + x*y + y^2, x^2 - x*y + y^2]

Factor \({y^6} * {x^6}\) for factors containing x.

syms x y
F = factor(y^2*x^2,x)

F =  [ y^2, x, x]

factor combines all factors without x into the first element. The remaining elements of F contain irreducible factors that contain x.

Factor the polynomial y for factors containing symbolic variables b and c.

syms a b c d
y = -a*b^5*c*d*(a^2 - 1)*(a*d - b*c);
F = factor(y,[b c])

F =  [ -a*d*(a - 1)*(a + 1), b, b, b, b, b, c, a*d - b*c]

factor combines all factors without b or c into the first element of F. The remaining elements of F contain irreducible factors of y that contain either b or c.

Simplify these symbolic expressions:

syms a
Z = a + 2*a + (a-3)^2 - (a^2 + 2*a +1)
simplify(Z)

Z = a + (a-3)^2 - a^2 - 1
ans = 8 - 5*a

syms x a b c
S1 = simplify(sin(x)^2 + cos(x)^2)
S2 = simplify(exp(c*log(sqrt(a+b))))

S1 = 1
S2 = (a + b)^(c/2)

Call simplify for this symbolic matrix. When the input argument is a vector or matrix, simplify tries to find a simpler form of each element of the vector or matrix.

syms x
M = [(x^2 + 5*x + 6)/(x + 2), sin(x)*sin(2*x) + cos(x)*cos(2*x);(exp(-x*i)*i)/2 - (exp(x*i)*i)/2, sqrt(16)];
S = simplify(M)

S =
[ x + 3, cos(x)]
[ sin(x), 4]

Find the numerator and denominator of a symbolic number.

[n, d] = numden(sym(4/5))

n = 4
d = 5

Find the numerator and denominator of the symbolic expression.

syms x y
F1 = numden((x-5)/(x+5))
[n,d] = numden(x/y + y/x)

F1 = x - 5
n = x^2 + y^2
d = x*y

Find the numerator and denominator of each element of a symbolic matrix.

syms a b
[n,d] = numden([a/b, 1/b; 1/a, 1/(a*b)])

n =
[ a, 1]
[ 1, 1]

d =
[ b, b]
[ a, a*b]

When used with an expression, the solve function sets the expression equal to zero and solves for the roots.

syms x
E1 = x-3;
solve(E1)

ans = 3

Use the == operator to specify the equation sin(x) == 1 and solve it.

syms x
eqn = sin(x) == 1;
solx = solve(eqn,x)

solx = pi/2

Solve the quadratic equation without specifying a variable to solve for. solve chooses x to return the solution.

syms a b c x
eqn = a*x^2 + b*x + c == 0;
S = solve(eqn)

S =
-(b + (b^2 - 4*a*c)^(1/2))/(2*a)
-(b - (b^2 - 4*a*c)^(1/2))/(2*a)

Specify the variable to solve for and solve the quadratic equation for a.

Sa = solve(eqn,a)

Sa =
-(c + b*x)/x^2

Solve the function for variable t.

syms P P0 r t
e3 = P==P0*exp(r*t);
solve(e3,t)

ans = log(P/P0)/r

To solve an expression set equal to something besides zero, you must use one of two approaches. If the equation is simple, you can transform it into an expression by subtracting the right-hand side from the left-hand side. For example,

\(5{x^2} + 6x + 3 = 10\)

could be reformulated as

\(5{x^2} + 6x - 7 = 0\)

syms x
E1 = 5*x^2 + 6*x - 7;
E2 = 5*x^2 + 6*x +3 == 10;
S1 = solve(E1)
S2 = solve(E2)

S1 =
- (2*11^(1/2))/5 - 3/5
(2*11^(1/2))/5 - 3/5
S2 =
- (2*11^(1/2))/5 - 3/5
(2*11^(1/2))/5 - 3/5

Notice that in both cases, the results are expressed as simply as possible, using fractions. You can use double function to convert a symbolic representation to a double-precision floating-point number:

double(s2)

ans =
-1.9266
0.7266

Solve the three embedded variables x , y , and z.

syms x y z
one = 3*x + 2*y -z == 10;
two = -x + 3*y + 2*z == 5;
three = x - y - z == -1;

List all three equations in the solve function:

answer = solve(one,two,three)

answer = struct with fields:
x: [1×1 sym]
y: [1×1 sym]
z: [1×1 sym]

To access the actual values, you’ll need to use the structure array syntax:

x=answer.x
y=answer.y
z=answer.z

x = -2
y = 5
z = -6

To force the results to be displayed without using a structure array and the associated syntax, we must assign names to the individual variables.

[X,Y,Z] = solve(one,two,three)

X = -2
Y = 5
Z = -6