Vol. 2 · Correspondences that move

Functions and Functional Equations

A companion to 《函数与函数方程》 by Xiong Bin, Zhu Zhen, Su Yong · 17 stops ·

Vol. 1Vol. 2Vol. 3Vol. 4Vol. 5Vol. 6Vol. 7Vol. 8

  1. §1.1MappingsEvery element of A fires exactly one arrow. That is a mapping.ConceptM12
  2. §1.2FunctionsA function maps numbers to numbers; to move its graph, move every point.Concept
  3. §2.1Odd and Even FunctionsEven folds onto itself, odd survives a half-turn; any f is even + odd.Concept
  4. §2.2MonotonicityIncreasing means the right point is always higher; prove it via f(x₂) − f(x₁).Concept
  5. §2.3PeriodicityShift the graph by T onto itself and T is a period; two mirrors make one too.Concept
  6. §3.1Quadratic FunctionsDiscriminant, axis and end values together lock both roots in an interval.ConceptM11
  7. §3.2Power, Exponential and Logarithmic Functionsaˣ and logₐx mirror each other in y = x, but they need not meet on that mirror.Concept
  8. §3.3The Function f(x) = x + a²/xTwo terms with fixed product a² have least sum 2a, exactly when they are equal.Concept
  9. Ch. 4Maxima and Minima of FunctionsA max or min is a bound plus a point that reaches it. The methods find bounds.Skill
  10. Ch. 5Solving Problems by Constructing a FunctionRead an expression as a value of a function; let its properties do the work.MethodM9M18
  11. §6.1Iteration of a FunctionFeed a function's output back in, again and again. A cobweb shows the orbit.ConceptM10
  12. §6.2Finding the n-th IterateGuess and prove; use a fixed point; or build a bridge from f to a simpler g.SkillM13
  13. §6.3Applications of IterationRepeat one map and ask where the orbit goes. Fixed points, shrinking, cycles.MethodM19
  14. §7.1Functional Equations: SubstitutionRewrite the equation with x replaced until f(x) returns, then eliminate.SkillM1
  15. §7.2Functional Equations: Plugging in ValuesIt holds for all x and y, so plug in the values that tell you most about f.SkillM15
  16. §7.3Functional Equations: Cauchy's MethodSolve on ℕ, climb to ℤ and ℚ, then reach ℝ with monotonicity or continuity.MethodM2
  17. §7.4Functional Equations: RecursionOn the positive integers, a functional equation is just a recurrence. Solve it.MethodM13