# Nesterov looks before correcting

Subject: Deep Learning

Exam: TEE

Reader: https://deep-learning-study-room.vercel.app/#tee/read/5/tee-c5-s10

Canonical: https://deep-learning-study-room.vercel.app/read/dl/tee/section/tee-c5-s10/

**Definition.** Nesterov accelerated gradient is a momentum-based method that evaluates the gradient at a lookahead parameter, formed using the previous velocity, before calculating the new velocity and actual parameter update.

### What changes relative to momentum

Ordinary momentum evaluates J′ at θₜ₋₁. Nesterov evaluates it after the anticipated displacement βvₜ₋₁. This changes the point where the objective's slope is measured:



$$
\theta_{\mathrm{look},t}=\theta_{t-1}+\beta v_{t-1}
$$





$$
g_t=J'(\theta_{\mathrm{look},t})
$$





$$
v_t=\beta v_{t-1}-\eta g_t
$$





$$
\theta_t=\theta_{t-1}+v_t
$$



The lookahead point is an auxiliary evaluation point, not the new actual parameter. In this signed-displacement convention, the new velocity is added to θₜ₋₁.

![Locate the lookahead point using the old velocity; measure the gradient there; use that gradient to calculate the actual update.](https://deep-learning-study-room.vercel.app/assets/figures/v_nesterov.svg)

Diagram: Locate the lookahead point using the old velocity; measure the gradient there; use that gradient to calculate the actual update.. Labels: Nesterov checks the slope at the look-ahead point | Nesterov checks the slope at the look-ahead point | Use the signed-velocity convention shown in the chapter. | Current w | Look-ahead | Updated w | Move by βv\_old | Correct using gradient here | Look-ahead = w + βv\_old | Nesterov measures the gradient after the look-ahead, before choosing its final | correction.

Locate the lookahead point using the old velocity; measure the gradient there; use that gradient to calculate the actual update. Tap the diagram to enlarge.

### Complete numerical: two iterations

**Question.** J(θ) = ½θ², θ₀ = 1, v₀ = 0, η = 0.1 and β = 0.9. Perform two Nesterov updates using signed displacement velocity.

### Given

J(θ) = ½θ²; θ₀ = 1; v₀ = 0.

η = 0.1; β = 0.9; signed-displacement velocity.

### To find

Find the gradient, velocity and actual parameter after each of two iterations.

### Formula — which one and why

**Nesterov lookahead.** First look ahead using the old velocity. Measure the gradient there, form the new velocity, then add that velocity to the original current parameter.

θ\_look = temporary gradient-evaluation point; v = signed displacement; β = momentum coefficient; η = learning rate.



$$
\theta_{\mathrm{look}}=\theta_{t-1}+\beta v_{t-1}
$$





$$
g_t=J\prime(\theta_{\mathrm{look}})
$$





$$
v_t=\beta v_{t-1}-\eta g_t
$$





$$
\theta_t=\theta_{t-1}+v_t
$$



**Yaad rakhne ka cue:** Pehle look ahead; gradient wahan; update original position se.

### Solution — step by step

The derivative is J′(θ) = θ. Therefore the gradient equals the lookahead value in this particular objective.

**Iteration 1 — step 1: locate the lookahead point.**



$$
\theta_{\mathrm{look},1}=1+0.9(0)=1
$$



**Step 2: evaluate the gradient there.**



$$
g_1=J'(1)=1
$$



**Step 3: calculate the new velocity and actual parameter.**



$$
v_1=0.9(0)-0.1(1)=-0.1
$$





$$
\theta_1=1+(-0.1)=0.9
$$



**Iteration 2 — step 1: use the previous actual parameter and velocity.**



$$
\theta_{\mathrm{look},2}=0.9+0.9(-0.1)=0.81
$$



**Step 2: evaluate the gradient at 0.81, not at 0.9.**



$$
g_2=J'(0.81)=0.81
$$



**Step 3: calculate the new velocity.**



$$
v_2=0.9(-0.1)-0.1(0.81)=-0.171
$$



**Step 4: update the old actual parameter.**



$$
\theta_2=0.9+(-0.171)=0.729
$$



| Iteration | Old actual θ | Lookahead θ | Gradient | New v | New actual θ |
| --- | --- | --- | --- | --- | --- |
| 1 | 1 | 1 | 1 | −0.1 | 0.9 |
| 2 | 0.9 | 0.81 | 0.81 | −0.171 | 0.729 |

**Final answer:** θ₁ = 0.9 and θ₂ = 0.729. The first update matches ordinary momentum because v₀ = 0. At the second update, ordinary momentum would use gradient 0.9 and produce θ₂ = 0.72. Adding v₂ to 0.81 instead of 0.9 would count the old momentum contribution twice.