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经济学
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需求价格弹性
Price elasticity of demand
中点弧弹性 E=%ΔQ/%ΔP · 弹性分类 · 对总收入影响
Midpoint (arc) elasticity between two points · effect on total revenue
比较优势(李嘉图模型)
Comparative advantage
机会成本 · 两国两品专业化 · 贸易收益 · PPF
Opportunity cost decides who should specialise — even if one country is better at everything
税收的无谓损失
Deadweight loss of a tax
税收楔子 · CS/PS/税收 · DWL 三角=½·t·ΔQ
A per-unit tax drives a wedge between buyers' and sellers' prices — the lost trades are the deadweight loss
拉弗曲线 · 税收与税率
Laffer curve · revenue vs. tax rate
税率-税收倒 U · 最优税率 t* · 0%/100% 皆零
R(t) = B0·t·(1−t)^k — an inverted U with a revenue-maximizing rate t* = 1/(1+k)
生产可能性边界 · 机会成本
Production Possibilities Frontier · opportunity cost
机会成本 · 有效/低效/不可行 · 边际转换率
Move the production point; see efficiency and the marginal rate of transformation
支出法 GDP · C + I + G + (X − M)
GDP by expenditure · C + I + G + (X − M)
C+I+G+(X−M) · 构成占比 · 净出口
Add up all final spending; net exports can be negative in a trade deficit
边际效用递减 · 总效用与边际效用曲线
Marginal utility · diminishing returns
总效用/边际效用递减 · MU=ΔTU/ΔQ · 效用最大化
Total utility rises at a decreasing rate; marginal utility is its slope, falling through zero
Phillips curve
Phillips curve
通胀↔失业短期权衡 · 长期垂直于自然失业率
The short-run inflation–unemployment tradeoff and the vertical long-run natural rate
Crowding-out effect
Crowding-out effect
政府借款推高利率 · 私人投资下降 · 部分抵消财政刺激
Government borrowing bids up the real interest rate, so private investment falls — offsetting part of the fiscal stimulus
Market for loanable funds
Market for loanable funds
储蓄供给+投资需求 · 定实际利率与资金量 · 盈余/短缺
Saving supplies funds, investment demands them — their crossing sets the real interest rate r*
The money multiplier
The money multiplier
部分准备金 · 乘数1/rr · 逐轮存贷派生 · 收敛
Fractional-reserve money creation: an initial deposit D expands the money supply by 1/rr
The quantity theory of money
The quantity theory of money
MV=PQ · P=MV/Q · 货币增长→通胀
The equation of exchange MV = PQ, and why money growth becomes inflation
The Keynesian spending multiplier
The Keynesian spending multiplier
1/(1−MPC) · 逐轮支出等比数列 · ΔG→ΔY
An injection ΔG raises output by ΔG/(1−MPC): each round spends a fraction MPC of the last
The business cycle & the output gap
The business cycle & the output gap
扩张/顶峰/衰退/谷底 · 绕潜在GDP波动 · 产出缺口
Actual output oscillates around the long-run trend (potential GDP): expansion → peak → recession → trough → recovery
Okun's law: unemployment gap vs output gap
Okun's law: unemployment gap vs output gap
失业缺口↔产出缺口 · 约1%失业↔2%GDP · 线性
output gap % = −c · (u − u*), with the Okun coefficient c ≈ 2
The Consumer Price Index & the inflation rate
The Consumer Price Index & the inflation rate
一篮子 · CPI=当期/基期×100 · 通胀率=ΔCPI/CPI₀
CPI = (cost of a fixed basket now ÷ cost in the base year) · 100; inflation % = (CPI_this − CPI_last) / CPI_last · 100
Solow growth model
Solow growth model
Δk=s·f(k)−(n+δ)k · 稳态k*=(s/(n+δ))^(1/(1−α)) · 收敛
Δk = s·f(k) − (n+δ)·k with f(k) = k^α: the steady state k* is where saving meets break-even investment
The foreign-exchange market
The foreign-exchange market
货币供求定汇率 · 需求增→升值 · 供给增→贬值
The exchange rate is a price: demand for and supply of the currency set it, and shifts appreciate or depreciate it
Consumer & producer surplus
Consumer & producer surplus
均衡处 CS+PS 三角 · 总福利
CS is the area under demand above price; PS is the area above supply below price; total welfare is CS + PS