Split View: AI를 위한 수학 완전 가이드 — 선형대수부터 정보이론까지

import numpy as np

# A single neuron = dot product
weights = np.array([0.5, -0.3, 0.8])  # weights
inputs = np.array([1.0, 2.0, 3.0])     # inputs
bias = 0.1

output = np.dot(weights, inputs) + bias
# 0.5*1.0 + (-0.3)*2.0 + 0.8*3.0 + 0.1 = 2.5

# An entire layer = matrix multiplication
W = np.random.randn(4, 3)  # 3 → 4 neurons
x = np.random.randn(3)      # input vector
h = W @ x                    # matrix-vector product = layer output

# Representing words as vectors (Word Embedding)
king = np.array([0.8, 0.2, 0.9, -0.5])
queen = np.array([0.7, 0.8, 0.85, -0.4])
man = np.array([0.9, 0.1, 0.5, -0.6])
woman = np.array([0.8, 0.7, 0.45, -0.5])

# king - man + woman ≈ queen (the famous relationship!)
result = king - man + woman
print(f"king - man + woman = {result}")
print(f"queen              = {queen}")
# Nearly identical!

# Cosine similarity — how similar two vectors are
def cosine_similarity(a, b):
    return np.dot(a, b) / (np.linalg.norm(a) * np.linalg.norm(b))

print(cosine_similarity(result, queen))  # ~0.95 (very similar!)

# Transformer's Self-Attention is also matrix multiplication!
# Q, K, V = input multiplied by weight matrices
batch_size, seq_len, d_model = 2, 10, 64

X = np.random.randn(batch_size, seq_len, d_model)
W_Q = np.random.randn(d_model, d_model)
W_K = np.random.randn(d_model, d_model)
W_V = np.random.randn(d_model, d_model)

Q = X @ W_Q  # Query: (2, 10, 64)
K = X @ W_K  # Key:   (2, 10, 64)
V = X @ W_V  # Value: (2, 10, 64)

# Attention Score = Q x K^T / sqrt(d)
scores = Q @ K.transpose(0, 2, 1) / np.sqrt(d_model)
# scores shape: (2, 10, 10) — attention each token pays to other tokens

# PCA: Finding the principal directions of data
from sklearn.decomposition import PCA

# 100-dimensional data → reduced to 2 dimensions
data = np.random.randn(1000, 100)
pca = PCA(n_components=2)
reduced = pca.fit_transform(data)

# What happens internally:
# 1. Compute covariance matrix: C = X^T X / n
# 2. Eigenvalue decomposition: C = V Lambda V^T
# 3. Select eigenvectors corresponding to largest eigenvalues
# → The directions of greatest data variance!

# SVD (Singular Value Decomposition) — Used for LLM compression!
# LoRA is exactly this: decomposing a large matrix into 2 small matrices

W = np.random.randn(768, 768)  # GPT-2's attention weight

# SVD: W = U x Sigma x V^T
U, S, Vt = np.linalg.svd(W)

# Keep only the top r values for approximation (the principle behind LoRA!)
r = 16  # rank
W_approx = U[:, :r] @ np.diag(S[:r]) @ Vt[:r, :]

# Original: 768x768 = 589,824 parameters
# LoRA: 768x16 + 16x768 = 24,576 parameters (only 4%!)
error = np.linalg.norm(W - W_approx) / np.linalg.norm(W)
print(f"Rank-{r} approximation error: {error:.4f}")

# f(x, y) = x^2 + 2xy + y^2
# df/dx = 2x + 2y (treating y as constant)
# df/dy = 2x + 2y (treating x as constant)

def f(x, y):
    return x**2 + 2*x*y + y**2

def df_dx(x, y):
    return 2*x + 2*y  # gradient in x direction

def df_dy(x, y):
    return 2*x + 2*y  # gradient in y direction

# Gradient vector
gradient = np.array([df_dx(3, 2), df_dy(3, 2)])
print(f"nabla f(3,2) = {gradient}")  # [10, 10]
# → Moving in the opposite direction decreases the function value!

# y = f(g(x)) → dy/dx = df/dg x dg/dx

# In neural networks:
# Loss = CrossEntropy(softmax(Wx + b), target)
# dLoss/dW = dLoss/dsoftmax x dsoftmax/d(Wx+b) x d(Wx+b)/dW

# PyTorch does this automatically!
import torch

x = torch.tensor([1.0, 2.0, 3.0], requires_grad=True)
W = torch.randn(2, 3, requires_grad=True)
b = torch.randn(2, requires_grad=True)

# Forward
h = W @ x + b
loss = h.sum()

# Backward (chain rule applied automatically!)
loss.backward()

print(f"dLoss/dW = {W.grad}")  # automatic differentiation!
print(f"dLoss/db = {b.grad}")
print(f"dLoss/dx = {x.grad}")

# Loss function: L(w) = (w - 3)^2
# Minimum: w = 3

def loss(w):
    return (w - 3) ** 2

def grad(w):
    return 2 * (w - 3)

# Gradient descent
w = 10.0  # starting point (way off)
lr = 0.1  # learning rate

for step in range(20):
    g = grad(w)
    w = w - lr * g  # move in the opposite direction of the gradient!
    if step % 5 == 0:
        print(f"Step {step}: w={w:.4f}, loss={loss(w):.4f}")

# Step 0:  w=8.6000, loss=31.3600
# Step 5:  w=3.2150, loss=0.0462
# Step 10: w=3.0070, loss=0.0000
# Step 15: w=3.0002, loss=0.0000
# → w converges to 3!

If lr is too large:
  w: 10 → -4 → 18 → -22 → diverges!

If lr is too small:
  w: 10 → 9.86 → 9.72 → ... → after 1M steps → 3.001

With the right lr:
  w: 10 → 8.6 → 7.48 → ... → after 20 steps → 3.0002

# Adam = Momentum + RMSprop (modern deep learning standard)
class Adam:
    def __init__(self, params, lr=0.001, beta1=0.9, beta2=0.999, eps=1e-8):
        self.lr = lr
        self.beta1 = beta1  # momentum (inertia)
        self.beta2 = beta2  # moving average of squared gradients
        self.eps = eps
        self.m = {id(p): 0 for p in params}  # 1st moment
        self.v = {id(p): 0 for p in params}  # 2nd moment
        self.t = 0

    def step(self, params, grads):
        self.t += 1
        for p, g in zip(params, grads):
            pid = id(p)
            # Momentum: remember previous gradient direction
            self.m[pid] = self.beta1 * self.m[pid] + (1 - self.beta1) * g
            # Adaptive learning rate: adjust based on gradient magnitude
            self.v[pid] = self.beta2 * self.v[pid] + (1 - self.beta2) * g**2
            # Bias correction
            m_hat = self.m[pid] / (1 - self.beta1**self.t)
            v_hat = self.v[pid] / (1 - self.beta2**self.t)
            # Update
            p -= self.lr * m_hat / (np.sqrt(v_hat) + self.eps)

# GPT's output = probability distribution of the next token
logits = np.array([2.0, 1.0, 0.1, -1.0, 3.0])  # model output (raw)
vocab = ["the", "cat", "sat", "on", "mat"]

# Softmax: logits → probabilities
def softmax(x):
    exp_x = np.exp(x - np.max(x))  # numerical stability
    return exp_x / exp_x.sum()

probs = softmax(logits)
for word, p in zip(vocab, probs):
    print(f"  {word}: {p:.4f}")
# the: 0.2312, cat: 0.0851, sat: 0.0346, on: 0.0115, mat: 0.6276
# → "mat" has the highest probability!

# P(A|B) = P(B|A) x P(A) / P(B)
# "The probability that the model is correct given the data"

# Example: Spam filter
# P(spam|"free") = P("free"|spam) x P(spam) / P("free")
p_free_given_spam = 0.8    # probability of "free" appearing in spam
p_spam = 0.3               # proportion of all emails that are spam
p_free = 0.35              # probability of "free" appearing in all emails

p_spam_given_free = (p_free_given_spam * p_spam) / p_free
print(f"P(spam|'free') = {p_spam_given_free:.2f}")  # 0.69 (69%!)

# Gaussian (Normal) Distribution — The core of Diffusion Models!
def gaussian(x, mu=0, sigma=1):
    return (1 / (sigma * np.sqrt(2 * np.pi))) * np.exp(-0.5 * ((x - mu) / sigma) ** 2)

# DDPM (Image Generation):
# Forward:  clean image → add Gaussian noise (gradually destroy)
# Reverse:  noise → remove noise (neural network learns this) → clean image!

# Noise addition process
def add_noise(image, t, noise_schedule):
    """x_t = sqrt(alpha_bar_t) x x_0 + sqrt(1 - alpha_bar_t) x epsilon"""
    alpha_bar = noise_schedule[t]
    noise = np.random.randn(*image.shape)  # Gaussian noise
    noisy = np.sqrt(alpha_bar) * image + np.sqrt(1 - alpha_bar) * noise
    return noisy, noise

# Measures how different the model's predictions are from the ground truth
def cross_entropy(predictions, targets):
    """H(p, q) = -sum p(x) log q(x)"""
    return -np.sum(targets * np.log(predictions + 1e-9))

# Ground truth: "cat" (one-hot encoding)
target = np.array([0, 1, 0, 0, 0])  # [the, cat, sat, on, mat]

# Good prediction
good_pred = np.array([0.05, 0.85, 0.03, 0.02, 0.05])
print(f"Good: {cross_entropy(good_pred, target):.4f}")  # 0.1625 (low)

# Bad prediction
bad_pred = np.array([0.3, 0.1, 0.2, 0.2, 0.2])
print(f"Bad:  {cross_entropy(bad_pred, target):.4f}")  # 2.3026 (high)

def entropy(probs):
    """H(X) = -sum p(x) log2 p(x)"""
    return -np.sum(probs * np.log2(probs + 1e-9))

# Fair coin: H = 1 bit (maximum uncertainty)
fair_coin = np.array([0.5, 0.5])
print(f"Fair coin: {entropy(fair_coin):.2f} bits")  # 1.00

# Biased coin: H is less than 1 bit (predictable)
biased_coin = np.array([0.9, 0.1])
print(f"Biased coin: {entropy(biased_coin):.2f} bits")  # 0.47

# Low entropy in GPT's output → the model is confident
# Raising temperature → entropy increases → more diverse outputs

def kl_divergence(p, q):
    """D_KL(P || Q) = sum p(x) log(p(x) / q(x))"""
    return np.sum(p * np.log(p / (q + 1e-9) + 1e-9))

# In VAE (Variational Autoencoder):
# Minimize KL(q(z|x) || p(z))
# = Make the encoder's output distribution close to standard normal!

# In RLHF:
# Add KL(pi_new || pi_ref) as penalty
# = Prevent the new model from deviating too far from the reference model!

[Week 1] Linear Algebra Fundamentals
  → Vectors, matrix multiplication, transpose, inverse
  → Implement from scratch with numpy

[Week 2] Calculus + Optimization
  → Partial derivatives, chain rule, gradient descent
  → Understand PyTorch autograd

[Week 3] Probability + Statistics
  → Conditional probability, Bayes, distributions
  → Implement softmax, cross-entropy

[Week 4] Information Theory + Practice
  → Entropy, KL-divergence
  → Find the math in nanoGPT/DDPM code