Deep Learning · MT2 · formulas

Adam: both memories

Adam keeps two separate running averages. The first remembers signed gradients and can be negative. The second remembers squared gradients and is nonnegative. These stored averages are not weights, and they are not extra gradients to multiply together.

$m_t=\beta_1m_{t-1}+(1-\beta_1)g_t$ LaTeX: m_t=\beta_1m_{t-1}+(1-\beta_1)g_t

$v_t=\beta_2v_{t-1}+(1-\beta_2)g_t^2$ LaTeX: v_t=\beta_2v_{t-1}+(1-\beta_2)g_t^2

Optimizer notation: old value, memory, new value

1. A parameter and a stored state are different

θ is the parameter we want to learn. A memory such as v, s or m helps an optimizer decide how to change θ; it is not a second prediction. Each trainable parameter usually has its own corresponding memory entries.

The time label tells you which value to use. Carry the previous memory into the current update. Tap the diagram to enlarge.

At time t = 0, the problem supplies starting values. At update t = 1, read the old values, calculate the current gradient and new memory, then calculate the new parameter. At update t = 2, use the results of update 1 as the old values.

2. A supplied gradient versus a supplied objective

If the question says gradients are 2 and −1, use those values in order. If it instead gives J(θ) = θ², first differentiate to get g(θ) = 2θ. Recalculate g using the current parameter at each iteration. Nesterov evaluates at its lookahead point instead.

For θ₀ = 1 and η = 0.1 with ordinary GD, g₁ = 2 and θ₁ = 0.8. The next gradient is 2 × 0.8 = 1.6, not the old value 2. Therefore θ₂ = 0.8 − 0.1 × 1.6 = 0.64.

3. A squared gradient keeps size and loses sign

Both 2² and (−2)² equal 4. Squared-gradient memory tells us about scale. The signed numerator still decides the update direction. Never replace the gradient in every part of an optimizer by its square.

4. Square root and epsilon belong to the denominator

If s = 4 and ε = 0.01, the denominator √s + ε is 2 + 0.01 = 2.01. It is different from √(s + ε), which is √4.01. Use the placement specified by the formula. Small ε keeps a zero accumulator from causing division by zero.

5. A table prevents mixing iterations

Write one row per iteration. Ordinary GD needs current θ, gradient and new θ. Momentum adds old and new velocity. Adam adds both memories and their corrected values. Complete a row before starting the next; otherwise a new value from iteration 2 may accidentally be used inside iteration 1.

The worked examples below use this exact order, with every state shown.

Adam remembers both direction and squared size

Definition. Adam, or adaptive moment estimation, uses bias-corrected exponentially weighted estimates of the first moment and the uncentered second moment of gradients to calculate parameter-specific updates.

What the two states represent

State or coefficient	Meaning
mₜ	Moving average of signed gradients; retains directional information
sₜ	Moving average of squared gradients; estimates their uncentered second moment
β₁	Decay coefficient for the first-moment estimate
β₂	Decay coefficient for the second-moment estimate
t	Update number, starting at 1 for the first update
η and ε	Base learning rate and small denominator stabilizer

The second moment is not the variance: it averages g² without subtracting a mean. Both states start at zero unless specified otherwise. These notes use s throughout this lesson; texts that use v for Adam's second moment mean the same state, not momentum's displacement velocity.

The current gradient feeds two distinct estimates. Correct both estimates before combining direction and scale in the parameter update. Tap the diagram to enlarge.

Recurrences, bias correction and update

$m_t=\beta_1m_{t-1}+(1-\beta_1)g_t$ LaTeX: m_t=\beta_1m_{t-1}+(1-\beta_1)g_t

$s_t=\beta_2s_{t-1}+(1-\beta_2)g_t^2$ LaTeX: s_t=\beta_2s_{t-1}+(1-\beta_2)g_t^2

Zero initialization reduces the total weight initially attached to observed gradients. Divide by the appropriate accumulated observation weight:

$\hat m_t=\frac{m_t}{1-\beta_1^t}$ LaTeX: \hat m_t=\frac{m_t}{1-\beta_1^t}

$\hat s_t=\frac{s_t}{1-\beta_2^t}$ LaTeX: \hat s_t=\frac{s_t}{1-\beta_2^t}

The update uses the corrected signed average in the numerator and the root of the corrected squared-gradient average in the denominator:

$\theta_t=\theta_{t-1}-\frac{\eta\hat m_t}{\sqrt{\hat s_t}+\varepsilon}$ LaTeX: \theta_t=\theta_{t-1}-\frac{\eta\hat m_t}{\sqrt{\hat s_t}+\varepsilon}

The derivation in the next lesson explains why the correction factors contain powers of the iteration number. No additional multiplication by gₜ belongs in this final update: its contribution is already inside mₜ.

Complete numerical: changing gradient magnitude

Question. θ₀ = 1, m₀ = s₀ = 0, η = 0.1, β₁ = 0.9, β₂ = 0.999. Given g₁ = 2 and g₂ = 1, find two Adam updates, neglecting epsilon only in the numerical arithmetic.

Given

θ₀ = 1; initial state(s) = 0; η = 0.1.

Supplied gradients g₁ = 2, g₂ = 1.

ε = 0 for this hand calculation.

β₁ = 0.9; β₂ = 0.999; use bias correction.

To find

Calculate all required states and the parameter after each of two updates.

Formula — which one and why

Adam with bias correction. Keep two states: one averages signed gradients, the other averages squared gradients. Correct their initial bias toward zero before combining them in the parameter update.

m = signed average; s (also written v) = squared average; β₁ and β₂ = retained fractions; t = iteration number. A hat marks a corrected value. Carry raw m and s into the next recurrence.

$m_t=\beta_1m_{t-1}+(1-\beta_1)g_t$ LaTeX: m_t=\beta_1m_{t-1}+(1-\beta_1)g_t

$s_t=\beta_2s_{t-1}+(1-\beta_2)g_t^2$ LaTeX: s_t=\beta_2s_{t-1}+(1-\beta_2)g_t^2

$\hat m_t=\frac{m_t}{1-\beta_1^t}$ LaTeX: \hat m_t=\frac{m_t}{1-\beta_1^t}

$\hat s_t=\frac{s_t}{1-\beta_2^t}$ LaTeX: \hat s_t=\frac{s_t}{1-\beta_2^t}

$\theta_t=\theta_{t-1}-\frac{\eta\hat m_t}{\sqrt{\hat s_t}+\epsilon}$ LaTeX: \theta_t=\theta_{t-1}-\frac{\eta\hat m_t}{\sqrt{\hat s_t}+\epsilon}

Yaad rakhne ka cue: Direction memory + square memory; hats lagao, phir update.

Solution — step by step

Iteration 1 — step 1: update the two raw states.

$m_1=0.9(0)+0.1(2)=0.2$ LaTeX: m_1=0.9(0)+0.1(2)=0.2

Step 2: correct both states using t = 1.

$\hat m_1=\frac{0.2}{1-0.9^1}=\frac{0.2}{0.1}=2$ LaTeX: \hat m_1=\frac{0.2}{1-0.9^1}=\frac{0.2}{0.1}=2

Step 3: update the parameter.

$\theta_1=1-\frac{0.1(2)}{\sqrt4}=0.9$ LaTeX: \theta_1=1-\frac{0.1(2)}{\sqrt4}=0.9

Adam iteration 1: follow the two state calculations

Start with the current gradient and the two states saved from the previous iteration.

Current gradient2g1

Previous raw m0

Previous raw s0

Signed average m10.2

Bias-corrected m̂12

Squared-gradient average s10.004

Bias-corrected ŝ14

Combine: ηm̂ / √ŝ0.1ε is neglected for this specific numerical.

Old θ1

New θ0.9

Carry forward raw states: m = 0.2, s = 0.004. The corrected values are used for this update.

Iteration 2 — step 1: carry forward the raw states m₁ and s₁.

The recurrence uses the raw states 0.2 and 0.004, not the corrected values 2 and 4.

Step 2: correct both states using t = 2.

$\hat m_2=\frac{0.28}{0.19}\approx1.473684211$ LaTeX: \hat m_2=\frac{0.28}{0.19}\approx1.473684211

Step 3: divide the corrected first moment by the corrected root.

\frac{\eta\hat m_2}{\sqrt{\hat s_2}}\approx\frac{0.1(1.473684211)}{\sqrt{2.499249625}}\approx0.093217964

Step 4: update from θ₁.

Adam iteration 2: follow the two state calculations

Start with the current gradient and the two states saved from the previous iteration.

Current gradient1g2

Previous raw m0.2

Previous raw s0.004

Signed average m20.28

Bias-corrected m̂21.473684211

Squared-gradient average s20.004996

Bias-corrected ŝ22.499249625

Combine: ηm̂ / √ŝ0.093217964ε is neglected for this specific numerical.

Old θ0.9

New θ0.806782036

Carry forward raw states: m = 0.28, s = 0.004996. The corrected values are used for this update.

State	After iteration 1	After iteration 2
Raw first moment m	0.2	0.28
Raw second moment s	0.004	0.004996
Corrected first moment	2	1.473684211
Corrected second moment	4	2.499249625
Parameter θ	0.9	0.806782036

Final answer: θ₁ = 0.9 and θ₂ ≈ 0.806782. Store the raw moments for the next recurrence and use the corrected moments for the current parameter update.

Properties and limitations

Adam combines directional averaging with coordinate-specific scaling, which can be useful with noisy or differently scaled gradients. It requires two state values per parameter in addition to the parameter itself. η, β₁, β₂ and ε remain hyperparameters. Faster reduction of training loss does not establish better generalization, and Adam does not guarantee the global optimum of a general neural-network objective.

Drill 26 Adam variation 1

Question

Question. θ₀ = 1, η = 0.1, supplied successive gradients = [2, 1]. Use Adam for two updates, zero initial accumulators and ε = 0 for this hand calculation. Use β₁ = 0.9, β₂ = 0.999 and bias correction.