Power Series and Analyticity in the Complex Plane · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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How far a power series reaches

Mathematics · Complex Analysis · ages 20-21
Name ______________________   Date ____________
  1. For f holomorphic near a with f'''(a) = 18, what is the coefficient of (z - a)³ in its Taylor series about a?

    Answer: ______________

  2. The series sum over n ≥ 0 of n! z^n has coefficients growing without bound. What is its radius of convergence?

    • 0
    • 1
    • Infinite
  3. The series 1 + z + z² + ... sums to 1/(1 - z). For which z does it converge?

    • Everywhere in the plane
    • Inside the unit disc, |z| < 1
    • Only at zero itself
  4. The radius is 1 because the function 1/(1 + x²) blows up at the real point x = 1.

    Circle one:   True   False

  5. On |z| ≤ 1/2, what is the minimum truncation degree N guaranteeing the geometric tail is at most 1/8 in modulus?

    Answer: ______________

  6. The function 1/(1 + z²) is expanded about 1. What sets its radius of convergence?

    • Uncancelled poles at ±i, each at distance √2 from 1
    • Poles at ±i, each at distance 1 from the centre
    • Real denominator zeros at ±1, at distance 0
  7. For the series sum over n ≥ 1 of z^n/n, which boundary point converges?

    • z = 1
    • z = -1
    • Every point of |z| = 1
  8. A function analytic on the union of disjoint disks about 0 and 3, equal to 0 on the first and 1 on the second, shows that zeros accumulating in one component do not force the function to zero on the whole union. True or false?

    Circle one:   True   False

  9. f and g are analytic on the unit disk with f(1/n) = g(1/n) for every integer n ≥ 2, yet f'(0) = g'(0) + 1. Is this consistent?

    Circle one:   True   False

  10. f is analytic on the unit disk with f(1/n) = 1/n² for every integer n ≥ 2. What is f throughout the disk, and what is the order of its zero at 0?

    • f(z) = z², zero of order 2
    • f(z) = z², zero of order 1
    • Only the values on the sequence are known
LightMySky · lightmysky.comW1-mt_Mwxapy_pU8-s1

Answer key

For grown-ups. Fold this page away before handing over the rest.

How far a power series reaches W1-mt_Mwxapy_pU8-s1

  1. 3 · The coefficient is f'''(a)/3! = 18/6 = 3.
  2. 0 · The quotient (n+1)!/n! = n+1 tends to infinity, so R = 1/∞ = 0.
  3. Inside the unit disc, |z| < 1 · The terms tend to 0 exactly when the modulus is below 1.
  4. False · The function is calm at x = 1; the poles at ±i set the radius.
  5. 3 · The bound (1/2)^N ≤ 1/8 first holds at N = 3.
  6. Uncancelled poles at ±i, each at distance √2 from 1 · The uncancelled poles at ±i are each √2 from the centre 1; the wrong-centre and wrong-root options misstate the geometry.
  7. z = -1 · The alternating harmonic series converges at z = -1.
  8. True · The union is disconnected, so the identity theorem does not apply across components.
  9. False · The identity theorem forces f = g, contradicting the derivative claim.
  10. f(z) = z², zero of order 2 · The identity theorem forces f = z², which has a zero of order 2 at 0.
Worksheet · LightMySky