## Symbolic Summation

Symbolic Math Toolbox™ provides two functions for calculating sums:

• `sum` finds the sum of elements of symbolic vectors and matrices. Unlike the MATLAB® `sum`, the symbolic `sum` function does not work on multidimensional arrays. For details, follow the MATLAB `sum` page.

• `symsum` finds the sum of a symbolic series.

### Comparing `symsum` and `sum`

You can find definite sums by using both `sum` and `symsum`. The `sum` function sums the input over a dimension, while the `symsum` function sums the input over an index.

Consider the definite sum $S=\sum _{k=1}^{10}\frac{1}{{k}^{2}}.$ First, find the terms of the definite sum by substituting the index values for `k` in the expression. Then, sum the resulting vector using `sum`.

```syms k f = 1/k^2; V = subs(f, k, 1:10) S_sum = sum(V)```
```V = [ 1, 1/4, 1/9, 1/16, 1/25, 1/36, 1/49, 1/64, 1/81, 1/100] S_sum = 1968329/1270080```

Find the same sum by using `symsum` by specifying the index and the summation limits. `sum` and `symsum` return identical results.

`S_symsum = symsum(f, k, 1, 10)`
```S_symsum = 1968329/1270080```

### Computational Speed of `symsum` versus `sum`

For summing definite series, `symsum` can be faster than `sum`. For summing an indefinite series, you can only use `symsum`.

You can demonstrate that `symsum` can be faster than `sum` by summing a large definite series such as $S=\sum _{k=1}^{100000}{k}^{2}.$

To compare runtimes on your computer, use the following commands.

```syms k tic sum(sym(1:100000).^2); toc tic symsum(k^2, k, 1, 100000); toc```

### Output Format Differences Between `symsum` and `sum`

`symsum` can provide a more elegant representation of sums than `sum` provides. Demonstrate this difference by comparing the function outputs for the definite series $S=\sum _{k=1}^{10}{x}^{k}.$ To simplify the solution, assume ```x > 1```.

```syms x assume(x > 1) S_sum = sum(x.^(1:10)) S_symsum = symsum(x^k, k, 1, 10)```
```S_sum = x^10 + x^9 + x^8 + x^7 + x^6 + x^5 + x^4 + x^3 + x^2 + x S_symsum = x^11/(x - 1) - x/(x - 1)```

Show that the outputs are equal by using `isAlways`. The `isAlways` function returns logical `1` (`true`), meaning that the outputs are equal.

`isAlways(S_sum == S_symsum)`
```ans = logical 1```

For further computations, clear the assumptions.

`assume(x, 'clear')`

## Support

#### Mathematical Modeling with Symbolic Math Toolbox

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