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# taylor

## Syntax

``T = taylor(f,var)``
``T = taylor(f,var,a)``
``T = taylor(___,Name,Value)``

## Description

example

````T = taylor(f,var)` approximates `f` with the Taylor series expansion of `f` up to the fifth order at the point `var = 0`. If you do not specify `var`, then `taylor` uses the default variable determined by `symvar(f,1)`.```

example

````T = taylor(f,var,a)` approximates `f` with the Taylor series expansion of `f` at the point `var = a`.```

example

````T = taylor(___,Name,Value)` uses additional options specified by one or more `Name,Value` pair arguments. You can specify `Name,Value` after the input arguments in any of the previous syntaxes.```

## Examples

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Find the Maclaurin series expansions of the exponential, sine, and cosine functions up to the fifth order.

```syms x T1 = taylor(exp(x)) T2 = taylor(sin(x)) T3 = taylor(cos(x))```
```T1 = x^5/120 + x^4/24 + x^3/6 + x^2/2 + x + 1 T2 = x^5/120 - x^3/6 + x T3 = x^4/24 - x^2/2 + 1```

You can use the `sympref` function to modify the output order of symbolic polynomials. Redisplay the polynomials in ascending order.

```sympref('PolynomialDisplayStyle','ascend'); T1 T2 T3```
```T1 = 1 + x + x^2/2 + x^3/6 + x^4/24 + x^5/120 T2 = x - x^3/6 + x^5/120 T3 = 1 - x^2/2 + x^4/24```

The display format you set using `sympref` persists through your current and future MATLAB® sessions. Restore the default value by specifying the `'default'` option.

`sympref('default');`

Find the Taylor series expansions at x = 1 for these functions. The default expansion point is 0. To specify a different expansion point, use `ExpansionPoint`:

```syms x T = taylor(log(x), x, 'ExpansionPoint', 1)```
```T = x - (x - 1)^2/2 + (x - 1)^3/3 - (x - 1)^4/4 + (x - 1)^5/5 - 1```

Alternatively, specify the expansion point as the third argument of `taylor`:

`T = taylor(acot(x), x, 1)`
```T = pi/4 - x/2 + (x - 1)^2/4 - (x - 1)^3/12 + (x - 1)^5/40 + 1/2```

Find the Maclaurin series expansion for ```f = sin(x)/x```. The default truncation order is 6. Taylor series approximation of this expression does not have a fifth-degree term, so `taylor` approximates this expression with the fourth-degree polynomial:

```syms x f = sin(x)/x; T6 = taylor(f, x)```
```T6 = x^4/120 - x^2/6 + 1```

Use `Order` to control the truncation order. For example, approximate the same expression up to the orders 8 and 10:

```T8 = taylor(f, x, 'Order', 8) T10 = taylor(f, x, 'Order', 10)```
```T8 = - x^6/5040 + x^4/120 - x^2/6 + 1 T10 = x^8/362880 - x^6/5040 + x^4/120 - x^2/6 + 1```

Plot the original expression `f` and its approximations `T6`, `T8`, and `T10`. Note how the accuracy of the approximation depends on the truncation order.

```fplot([T6 T8 T10 f]) xlim([-4 4]) grid on legend('approximation of sin(x)/x up to O(x^6)',... 'approximation of sin(x)/x up to O(x^8)',... 'approximation of sin(x)/x up to O(x^{10})',... 'sin(x)/x','Location','Best') title('Taylor Series Expansion')```

Find the Taylor series expansion of this expression. By default, `taylor` uses an absolute order, which is the truncation order of the computed series.

`T = taylor(1/(exp(x)) - exp(x) + 2*x, x, 'Order', 5)`
```T = -x^3/3```

Find the Taylor series expansion with a relative truncation order by using `OrderMode`. For some expressions, a relative truncation order provides more accurate approximations.

`T = taylor(1/(exp(x)) - exp(x) + 2*x, x, 'Order', 5, 'OrderMode', 'relative')`
```T = - x^7/2520 - x^5/60 - x^3/3```

Find the Maclaurin series expansion of this multivariate expression. If you do not specify the vector of variables, `taylor` treats `f` as a function of one independent variable.

```syms x y z f = sin(x) + cos(y) + exp(z); T = taylor(f)```
```T = x^5/120 - x^3/6 + x + cos(y) + exp(z)```

Find the multivariate Maclaurin expansion by specifying the vector of variables.

```syms x y z f = sin(x) + cos(y) + exp(z); T = taylor(f, [x, y, z])```
```T = x^5/120 - x^3/6 + x + y^4/24 - y^2/2 + z^5/120 + z^4/24 + z^3/6 + z^2/2 + z + 2```

You can use the `sympref` function to modify the output order of a symbolic polynomial. Redisplay the polynomial in ascending order.

```sympref('PolynomialDisplayStyle','ascend'); T```
```T = 2 + z + z^2/2 + z^3/6 + z^4/24 + z^5/120 - y^2/2 + y^4/24 + x - x^3/6 + x^5/120```

The display format you set using `sympref` persists through your current and future MATLAB sessions. Restore the default value by specifying the `'default'` option.

`sympref('default');`

Find the multivariate Taylor expansion by specifying both the vector of variables and the vector of values defining the expansion point:

```syms x y f = y*exp(x - 1) - x*log(y); T = taylor(f, [x, y], [1, 1], 'Order', 3)```
```T = x + (x - 1)^2/2 + (y - 1)^2/2```

If you specify the expansion point as a scalar `a`, `taylor` transforms that scalar into a vector of the same length as the vector of variables. All elements of the expansion vector equal `a`:

`T = taylor(f, [x, y], 1, 'Order', 3)`
```T = x + (x - 1)^2/2 + (y - 1)^2/2```

## Input Arguments

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Input to approximate, specified as a symbolic expression or function. It also can be a vector, matrix, or multidimensional array of symbolic expressions or functions.

Expansion variable, specified as a symbolic variable. If you do not specify `var`, then `taylor` uses the default variable determined by `symvar(f,1)`.

Expansion point, specified as a number, or a symbolic number, variable, function, or expression. The expansion point cannot depend on the expansion variable. You also can specify the expansion point as a `Name,Value` pair argument. If you specify the expansion point both ways, then the `Name,Value` pair argument takes precedence.

### Name-Value Pair Arguments

Specify optional comma-separated pairs of `Name,Value` arguments. `Name` is the argument name and `Value` is the corresponding value. `Name` must appear inside quotes. You can specify several name and value pair arguments in any order as `Name1,Value1,...,NameN,ValueN`.

Example: `taylor(log(x),x,'ExpansionPoint',1,'Order',9)`

Expansion point, specified as a number, or a symbolic number, variable, function, or expression. The expansion point cannot depend on the expansion variable. You can also specify the expansion point using the input argument `a`. If you specify the expansion point both ways, then the `Name,Value` pair argument takes precedence.

Truncation order of Taylor series expansion, specified as a positive integer or a symbolic positive integer. `taylor` computes the Taylor series approximation with the order ```n - 1```. The truncation order `n` is the exponent in the O-term: O(varn).

Order mode indicator, specified as `'absolute'` or `'relative'`. This indicator specifies whether you want to use absolute or relative order when computing the Taylor polynomial approximation.

Absolute order is the truncation order of the computed series. Relative order `n` means that the exponents of `var` in the computed series range from the leading order `m` to the highest exponent ```m + n - 1```. Here `m + n` is the exponent of `var` in the O-term: O(varm + n).

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### Taylor Series Expansion

Taylor series expansion represents an analytic function f(x) as an infinite sum of terms around the expansion point x = a:

`$f\left(x\right)=f\left(a\right)+\frac{{f}^{\prime }\left(a\right)}{1!}\left(x-a\right)+\frac{{f}^{″}\left(a\right)}{2!}{\left(x-a\right)}^{2}+\dots =\sum _{m=0}^{\infty }\frac{{f}^{\left(m\right)}\left(a\right)}{m!}\cdot {\left(x-a\right)}^{m}$`

Taylor series expansion requires a function to have derivatives up to an infinite order around the expansion point.

### Maclaurin Series Expansion

Taylor series expansion around x = 0 is called Maclaurin series expansion:

`$f\left(x\right)=f\left(0\right)+\frac{{f}^{\prime }\left(0\right)}{1!}x+\frac{{f}^{″}\left(0\right)}{2!}{x}^{2}+\dots =\sum _{m=0}^{\infty }\frac{{f}^{\left(m\right)}\left(0\right)}{m!}{x}^{m}$`

## Tips

• If you use both the third argument `a` and `ExpansionPoint` to specify the expansion point, the value specified via `ExpansionPoint` prevails.

• If `var` is a vector, then the expansion point `a` must be a scalar or a vector of the same length as `var`. If `var` is a vector and `a` is a scalar, then `a` is expanded into a vector of the same length as `var` with all elements equal to `a`.

• If the expansion point is infinity or negative infinity, then `taylor` computes the Laurent series expansion, which is a power series in `1/var`.

• You can use the `sympref` function to modify the output order of symbolic polynomials.