# The difference between gbdt and XGB, and the mathematical derivation of gradient descent method and Newton method

2020-11-06 01:28:18

## Why the gradient descent method and Newton method are introduced ？

Two algorithms are mentioned here GBDT and XGBoost, All two are boosting Model .

GBDT and xgb The objective function of is different , At the same time, aiming at the error function in the objective function L(θ) There are also differences in the way of fitting ：

• GBDT Let's use the first order Taylor expansion to expand two terms , Make an approximation
• xgboost Let's use the second order Taylor expansion to expand the three terms , Make an approximation
Words mean ,
• GBDT The gradient descent method is used to optimize in function space
• XGBoost Using Newton's method to optimize in function space
The final objective function only depends on the first and second derivatives of each data point on the error function .

The error function can be customized , Let's say the square loss function ： ( y i , y i ) = ( y i − y i ) 2 (yi,y^i) = (yi−y^i)2 , or logistic Loss function

More introductions and suggestions to listen to ：https://study.163.com/course/courseMain.htm?courseId=1006401020&share=2&shareId=400000000645014

### 1. The derivation of the gradient descent method

Gradient descent method is widely used in machine learning and deep learning , The general course or textbook will explain the gradient descent method in a visualized way （ Quadratic curve 、 Lower convex surface, etc ）, I think we all know how to use visualization to illustrate the effectiveness of gradient descent method . here , I'm not going to repeat this kind of figurative explanation . I use mathematical derivation to prove the effectiveness of the gradient descent method .

We should all know the Taylor expansion of function of one variable , The formula is as follows ：

f ( x ) = f ( x 0 ) + f ’ ( x 0 ) 1 ! ( x − x 0 ) + f ’ ’ ( x 0 ) 2 ! ( x − x 0 ) 2 + f ’ ’ ’ ( x 0 ) 3 ! ( x − x 0 ) 3 + … f(x)=f(x_0)+\frac{f’(x_0)}{1!}(x-x_0)+\frac{f’’(x_0)}{2!}(x-x_0)^2+\frac{f’’’(x_0)}{3!}(x-x_0)^3+…

Let's just take the first two terms of the formula on the right , That's one “ About equal to ” Result ：

f ( x ) = f ( x 0 ) + f ’ ( x 0 ) 1 ! ( x − x 0 ) f(x)=f(x_0)+\frac{f’(x_0)}{1!}(x-x_0)

remember ： Δ x = x − x 0 \Delta x=x-x_0 , The above formula becomes ：

f ( x ) = f ( x 0 ) + f ’ ( x 0 ) 1 ! Δ x f(x)=f(x_0)+\frac{f’(x_0)}{1!}\Delta x

Our goal is to make the following equation hold in the iterative process , In other words, it is hoped that the function value will gradually decrease during the iteration process , To describe in mathematical language is ： f ( x n + 1 ) ≤ f ( x n ) f(x_{n+1})\leq f(x_{n})

Easy to think of , It should be constructed ：

Δ x = − f ’ ( x 0 ) \Delta x=-f’(x_0)

here ：

f ( x ) = f ( x 0 ) − f ’ ( x 0 ) 2 f(x)=f(x_0)-f’(x_0)^2

Write it in iterative form ：

f ( x n + 1 ) = f ( x n ) − f ’ ( x n ) 2 f(x_{n+1})=f(x_{n})-f’(x_{n})^2

because f ’ ( x ) 2 ≥ 0 f’(x)^2\geq 0 , We have completed the proof of the effectiveness of gradient descent . The formula of parameter iterative updating summarized from the above steps is as follows ：

f ( x n + 1 ) = f ( x n ) − f ’ ( x n ) 2 f(x_{n+1})=f(x_{n})-f’(x_{n})^2

The above steps prove the effectiveness of gradient descent on a function of one variable . It is easy to generalize to multivariate functions . in addition , In multivariate functions , It can also be added that the gradient direction is the fastest direction of descent .

See ： You know Why does gradient descent find a minimum ？

### 2. Newton method

That's the gradient descent method , By the way, the derivation of Newton's method . Because Newton's method is also derived from Taylor expansion .

Keep watching Taylor unfold ：

f ( x ) = f ( x 0 ) + f ’ ( x 0 ) 1 ! ( x − x 0 ) + f ’ ’ ( x 0 ) 2 ! ( x − x 0 ) 2 + f ’ ’ ’ ( x 0 ) 3 ! ( x − x 0 ) 3 + … f(x)=f(x_0)+\frac{f’(x_0)}{1!}(x-x_0)+\frac{f’’(x_0)}{2!}(x-x_0)^2+\frac{f’’’(x_0)}{3!}(x-x_0)^3+…

still , Let's take the right front 2 term ：

f ( x ) = f ( x 0 ) + f ’ ( x 0 ) 1 ! ( x − x 0 ) f(x)=f(x_0)+\frac{f’(x_0)}{1!}(x-x_0)

Take derivatives on both sides of the equation ：

f ’ ( x ) = f ’ ( x 0 ) + f ’ ’ ( x 0 ) 1 ! ( x − x 0 ) f’(x)=f’(x_0)+\frac{f’’(x_0)}{1!}(x-x_0)

f ’ ( x ) = f ’ ( x 0 ) + f ’ ’ ( x 0 ) 1 ! Δ x f’(x)=f’(x_0)+\frac{f’’(x_0)}{1!}\Delta x

According to the nature of calculus , f ( x ) f(x) When you take the minimum , Yes f ’ ( x ) = 0 f’(x)=0 , Let's put this property into the equation above , Yes ：

0 = f ’ ( x 0 ) + f ’ ’ ( x 0 ) 1 ! Δ x 0=f’(x_0)+\frac{f’’(x_0)}{1!}\Delta x

Δ x = − f ’ ( x 0 ) f ’ ’ ( x 0 ) \Delta x=-\frac{f’(x_0)}{f’’(x_0)}

In this way, we get the parameter iterative updating formula of Newton method as follows ：

x n + 1 = x n − f ’ ( x n ) f ’ ’ ( x n ) x_{n+1}=x_{n}-\frac{f’(x_n)}{f’’(x_n)}

### 3. The difference between the gradient descent method and Newton's method

From the above proof process, we can see that , Although both gradient descent method and Newton method can be derived by Taylor expansion , But there's a little bit of a difference in the way the reasoning is based .

In practice , Newton method and gradient descent method are widely used in machine learning . The difference between the two is actually written in many blogs , such as ： gradient descent or Quasi Newton method ？

### 4. Quasi Newton method

In the parameter iterative updating formula of Newton method above , We can see f ’ ’ ( x 0 ) f’’(x_0) It's in the denominator . remember , The mathematical derivation above is a function of one variable , For multivariate functions , The existence of this denominator is equivalent to calculating Hessian The inverse of the matrix , It's very difficult and time-consuming . therefore , Many variations of Newton's algorithm have appeared , This kind of deformation is called quasi Newton algorithm .BFGS It's an iterative method to approximate the Hessian matrix . and BFGS We need to store the approximate Hessian matrix , So there's an improved version L-BFGS. 