 ## Calculus of Variations Demystified | by NAOKI | Oct, 2020

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Before discovering the shortest path, we’d like to have the ability to calculate the size of a path from A to B.

As the well-known Chinese proverb say, “a journey of a thousand miles begins with a single step”.

So, let’s calculate the size of a small passage and combine it from A to B. Image by creator

As proven within the above diagram, a small passage may be approximated by a straight line.

So, the full size of a path may be calculated by the next integration:

Since `A` and `B` are mounted factors, let’s outline them as `A = (x1, y1)` and `B = (x2, y2)`.

In the final step, `y1` and `y2` are dropped as they’re decided by `x1` and `x2`.

So, we’re integrating the sq. root from `x1` till `x2`.

Now we all know how you can calculate the size of a path from A to B.

We are going to seek out the `y = f(x)` that minimizes the trail size `S`.

## What is a practical?

Instead, we predict of many potential resolution traces from A to B. Image by creator

The blue line is `y = blue_line(x)`.

The crimson line is `y = red_line(x)`.

And so on.

But having a perform title for every line is just too tedious, so we name them collectively `y = y(x)`.

We assume of `y = y(x)` as any line between A and B which may very well be the blue line, the crimson line or else.

We can calculate the size of any line (given a specific `y(x)`) utilizing the components `S`.

In different phrases, `S` takes an occasion of `y(x)` and returns the size of the road.

So, `S` maps a perform `y(x)` to a price.

We name `S` a practical which is a perform of capabilities.

A practical is written with sq. brackets like `S[y]` which suggests the practical `S` takes a perform `y` and returns a price.

If we speak about capabilities of capabilities like this normal, there may very well be too many varieties of functionals, many of which will not be that helpful.

Since the practical optimization originates from the Physics which offers with positions and velocities, the next normal type is quite common and has many purposes (not solely in Physics but in addition in machine studying).

`J[y]` is a practical that takes a perform `y`.

`J` integrates `L` from `x1` till `x2`.

`L` is a perform of `x`, `y`, and `y'`.

`y'` is the by-product of `y` in phrases of `x`.

The above might look too summary.

Let’s apply it to our shortest path downside.

For the shortest path downside, `L` is outlined as follows:

We combine `L` between `x1` and `x2` to get the size of the trail.

In the shortest path downside, `L` doesn’t embody `x` and `y` however solely `y'`.

That is okay.

The level is that if we will resolve the optimization downside for the overall type of functionals, we will resolve many issues together with the shortest path downside.

## Functional variations

Do we generate many random capabilities and consider the trail size to seek out out the shortest path? Image by creator

We can map every path to a path size worth utilizing the practical `S`.

The query is how you can know if we truly discover the shortest path or not.

We can take another method since we already know that the shortest path exists.

We begin with the shortest path and barely modify it to see what form of situation is required for the shortest path.

Let’s name the shortest path perform `f(x)` that reduce `S`.

Here, we write `f(x)` as a substitute of `y(x)`.

The perform `f(x)` solves our downside.

`y(x)` is a candidate perform that will or might not resolve our downside just like the blue line and the crimson line.

When we give `y(x)` or `f(x)` to `S`, we write like `S[y]` and `S[f]` respectively.

`S[y]` offers the trail size following `y(x)` from A to B.

`S[f]` offers the trail size following `f(x)` from A to B which is the shortest path size.

If we add an arbitrary perform `η(x)`(eta of x) to `f(x)`, the next relationship is asserted:

The time period `ϵη` known as a variation of the perform `f`, which is denoted as follows:

`ϵ`(epsilon) is a small quantity in order that the variation can also be small.

We analyze how the practical adjustments after we transfer `ϵ`in the direction of zero.

To make it extra concrete, let’s apply a variation to the shortest path.

In the beneath diagram, the blue straight line is the shortest path `y = f(x)`.

A purple line is an instance of a variation `ϵη` added to `f` as in `y = f(x) + ϵη(x)`. Image by creator

`S[f + ϵη]` offers the size of the trail given by the perform `f + ϵη`.

Since `S[f]` is the shortest path size, the connection `S[f] <= S[f + ϵη]` is true for any variation.

When `ϵ` goes to zero, the purple line turns into the identical because the blue line because the variation vanishes.

Also, word that `η(x)` should be zero on the level A and B since any variation should embody the purpose A and B. (Remember we’re searching for the shortest line from A to B).

So, `η(x1) = 0` and `η(x2) = 0`.

Why can we take into consideration the variation?

The purpose is that the variation makes the practical optimization right into a perform optimization which we all know how you can resolve.

If we have a look at `S[f + ϵη]` very intently, we will see that it relies upon solely on `ϵ`.

`f(x)` is the perform that offers the minimal path line which is mounted.

`η(x)` is an arbitrary perform that’s mounted for a specific variation.

Therefore, the one variable in `S[f + ϵη]` is `ϵ`.

In quick, `S[f + ϵη]` is a perform of `ϵ`.

So, let’s write it as `S(ϵ)`.

`S(ϵ)` is the perform of `ϵ` that returns the trail size for the perform `y` that’s the resolution perform `f` plus a variation `ϵη`.

We simply want to unravel the minimization downside for the perform `S(ϵ)`.

Conveniently, we additionally know that `S` turns into the minimal when `ϵ` goes to zero.

So, the by-product of `S` with `ϵ` ought to be zero when `ϵ = 0`.

Here, the apostrophe (`‘`) is for the derivative of `S` by `ϵ`.

In case it’s not clear, an apostrophe is used after we calculate the by-product of a perform by the one impartial variable. Since `ϵ` is the one impartial variable of `S` on this context, the apostrophe right here means the by-product of `S` by `ϵ`.

Let’s return to the overall type to reiterate all of the factors we’ve mentioned up to now and derive the Euler-Lagrange equation to unravel the optimization downside.

## The Euler-Lagrange equation

We say the perform `f` minimizes `J`.

As such, `J[f]` is the minimal worth of the practical `J`.

We outline `y = f + ϵη` to imply we add a variation to the answer perform.

`y` ought to nonetheless fulfill the boundary situation

As such, `η(x1) = 0` and `η(x2) = 0`.

Since `J[f]` is the minimal (fixed) and `η` is an arbitrary perform of `x` which is mounted for a specific variation, we will outline `J[y]` as a perform of `ϵ`:

We’ve efficiently transformed the overall type of functionals right into a perform of `ϵ`.

This perform of `ϵ` is at minimal when `ϵ = 0`.

In different phrases, the primary by-product of it turns into zero at `ϵ = 0`.

So far, we bolstered the identical concept from the shortest path downside for the overall type of functionals.

Now, we’re going to resolve it for the overall type of functionals to derive the Euler-Lagrange equation.

By substituting `J[y]` into the above equation:

Let’s calculate the full by-product of `L(x, y, y')` by `ϵ`.

The first time period is zero since `x` has no dependency on `ϵ`.

`y = f + ϵη` and `y' = f' + ϵη'`.

`f`, `f'`, `η` and `η'` haven’t any dependency on `ϵ`.

So, the full by-product of `L` by `ϵ` is as follows:

When `ϵ=0`, `y = f` and `y' = f'`:

Substituting this into the equation `∅'(0)=0`.

In the second time period, now we have `η’`.

As `η` is an arbitrary perform, now we have no strategy to calculate `η’`.

Let’s remove `η'` by making use of integration by components on the second time period.

The first time period is zero since `η(x1) = 0` and `η(x2) = 0`.

Therefore,

As `η(x)` is an arbitrary perform, the time period within the brackets should be zero to fulfill the equation `∅'(0)=0`.

This known as the Euler-Lagrange equation which is the situation that should be glad for the optimum perform `f` of the overall type of functionals.

## Solving the shortest path downside

As talked about earlier than, `y'` is the by-product of `y` in phrases of `x`.

Now, we are saying `y = f(x)` is the perform that minimizes the trail size from A to B.

So, we predict of `f` as a substitute of `y` in `L`.

This `f` should fulfill the Euler-Lagrange equation.

Let’s resolve the Euler-Lagrange equation for the shortest path downside.

Since `f` doesn’t seem in `L`, the primary time period is zero.

As such, the second time period can also be zero.

We combine the above by `x`:

`C` is a continuing worth.

This means `f'` can also be a relentless worth.

It may be proven by taking the sq. of either side and re-arrange the equation:

Let `f' = a` and we get `f(x) = ax + b` which is a straight line.

Voilà!

If we put the boundary circumstances `f(x1) = y1` and `f(x2) = y2`, we get the values for `a` and `b`.

## The final not the least…

We derived the Euler-Lagrange equation for the overall type of functionals which should be glad for resolution capabilities.

Mathematically talking, the Euler-Lagrange equation is just not adequate situation for the practical minimal or most. It simply tells us the answer perform makes the practical stationary.

However, in lots of issues, we all know the answer would give the practical minimal or most.

In the shortest path downside, the utmost has no restrict, so we all know the answer to the Euler-Lagrange equation offers the minimal perform.

In case we need to test if the answer perform is for the utmost or minimal, we have to calculate the second by-product of `∅(ϵ)` by `ϵ` for `ϵ=0` the place `y = f + ϵη`.

Let’s do that for the shortest path downside:

The first by-product of `S(ϵ)` with `ϵ`:

The second by-product of `S(ϵ)` with `ϵ`:

This is already a optimistic worth, so we all know the Euler-Lagrange equation for the shortest path downside offers the minimal perform.

In idea, we should always test the worth of the second by-product when `ϵ=0` as a result of we’re speaking concerning the native minimal/most across the vary expressed by `ϵ`.

When `ϵ=0`, `y = f`:

All in all, the answer perform (the straight line) given by the Euler-Lagrange equation for the shortest path downside offers the minimal of the practical `S`.

Lastly, I assumed all of the derivatives are doable on this article. For instance, `y(x)` and `η(x)` should be differentiable by `x`.

I hope you could have a clearer concept concerning the calculus of variations now.

The Calculus of Variations (Bounded Rationality)

Introduction to Calculus of Variations (Faculty of Khan)

https://youtu.be/6HeQc7CSkZs

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