mathematics

Wielandt theorem

Wielandt theorem is a mathematics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Wielandt theorem rather than just read about it. In short: In mathematics, the Wielandt theorem characterizes the gamma function, defined for all complex numbers z {\displaystyle z} for which R e z > 0 {\displaystyle \mathrm {Re} \,z>0} by Γ ( z ) = ∫ 0 + ∞ t z − 1 e − t d t , {\displaystyle \Gamma (z)=\int _{0}^{+\infty }t^{z-1}\mathrm {e} ^{-t}\,\mathrm {d} t,} as the only function f {\displaystyle f} defined on the half-plane H := { z ∈ C : Re z > 0 } {\displaystyle H:=\…

Key takeaways

Wielandt theorem belongs to mathematics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Wielandt theorem to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Wielandt theorem from memory before moving on to harder problems.

Reference excerpt

In mathematics, the Wielandt theorem characterizes the gamma function, defined for all complex numbers z {\displaystyle z} for which R e z > 0 {\displaystyle \mathrm {Re} \,z>0} by

Γ ( z ) = ∫ 0 + ∞ t z − 1 e − t d t , {\displaystyle \Gamma (z)=\int _{0}^{+\infty }t^{z-1}\mathrm {e} ^{-t}\,\mathrm {d} t,}

as the only function f {\displaystyle f} defined on the half-plane H := { z ∈ C : Re z > 0 } {\displaystyle H:=\{z\in \mathbb {C} :\operatorname {Re} \,z>0\}} such that:

f {\displaystyle f} is holomorphic on H {\displaystyle H} ;

f ( 1 ) = 1 {\displaystyle f(1)=1} ;

f ( z + 1 ) = z f ( z ) {\displaystyle f(z+1)=z\,f(z)} for all z ∈ H {\displaystyle z\in H} and

f {\displaystyle f} is bounded on the strip { z ∈ C : 1 ≤ Re z ≤ 2 } {\displaystyle \{z\in \mathbb {C} :1\leq \operatorname {Re} \,z\leq 2\}} . This theorem is named after the mathematician Helmut Wielandt.

See also Bohr–Mollerup theorem Hadamard's gamma function

References Reinhold Remmert (1996). "Wielandt's theorem about the Γ-function". American Mathematical Monthly. 103: 214–220. JSTOR 2975370..

Worked examples

Example 1 — a first encounter with Wielandt theorem

Start with the simplest possible case. Write down what Wielandt theorem claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In mathematics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Wielandt theorem before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Wielandt theorem ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Wielandt theorem

In research: Wielandt theorem appears in mathematics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Wielandt theorem in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Wielandt theorem is common in secondary-school and first-year university syllabi. It links to neighbouring topics Gamma and related functions, Theorems in complex analysis, so understanding it makes those chapters shorter.
In everyday life: Look for Wielandt theorem outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Wielandt theorem in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Wielandt theorem means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Wielandt theorem out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Wielandt theorem in simple terms?

In mathematics, the Wielandt theorem characterizes the gamma function, defined for all complex numbers z {\displaystyle z} for which R e z > 0 {\displaystyle \mathrm {Re} \,z>0} by Γ ( z ) = ∫ 0 + ∞ t z − 1 e − t d t , {\displaystyle \Gamma (z)=\int _{0}^{+\infty }t^{z-1}\mathrm {e} ^{-t}\,\mathrm…

Why does Wielandt theorem matter?

Because it connects several mathematics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Wielandt theorem?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Wielandt theorem.