Landau kernel

The Landau kernel is named after the German number theorist Edmund Landau. The kernel is a summability kernel defined as:^[1]

$${\displaystyle L_{n}(t)={\begin{cases}{\frac {(1-t^{2})^{n}}{c_{n}}}&{\text{if }}{-1}\leq t\leq 1\\0&{\text{otherwise}}\end{cases}}}$$where the coefficients ${\displaystyle c_{n}}$ are defined as follows:

$${\displaystyle c_{n}=\int _{-1}^{1}(1-t^{2})^{n}\,dt.}$$

Visualisation

Using integration by parts, one can show that:^[2] $${\displaystyle c_{n}={\frac {(n!)^{2}\,2^{2n+1}}{(2n)!(2n+1)}}.}$$ Hence, this implies that the Landau kernel can be defined as follows: $${\displaystyle L_{n}(t)={\begin{cases}(1-t^{2})^{n}{\frac {(2n)!(2n+1)}{(n!)^{2}\,2^{2n+1}}}&{\text{for }}t\in [-1,1]\\0&{\text{elsewhere}}\end{cases}}}$$

Plotting this function for different values of n reveals that as n goes to infinity, ${\displaystyle L_{n}(t)}$ approaches the Dirac delta function, as seen in the image,^[1] where the following functions are plotted.

Properties

Some general properties of the Landau kernel is that it is nonnegative and continuous on ${\displaystyle \mathbb {R} }$. These properties are made more concrete in the following section.

Dirac sequences

Definition: Dirac sequence—A Dirac sequence is a sequence ${\displaystyle \{K_{n}(t)\}}$ of functions ${\displaystyle K_{n}(t)\colon \mathbb {R} \to \mathbb {R} }$ that satisfies the following properities:

• ${\displaystyle K_{n}(t)\geq 0,\,\,\forall t\in \mathbb {R} {\text{ and }}\forall n\in \mathbb {Z} }$
• ${\displaystyle \int _{-\infty }^{\infty }K_{n}(t)\,dt=1,\,\forall n}$
• ${\displaystyle \forall \varepsilon >0\,\forall \delta >0\,\exists N\in \mathbb {Z} _{+}\,\forall n\geq N:}$
  ${\displaystyle {}\quad \int _{\mathbb {R} \smallsetminus [-\delta ,\delta ]}K_{n}(t)\,dt=\int _{-\infty }^{-\delta }K_{n}(t)\,dt+\int _{\delta }^{\infty }K_{n}(t)\,dt<\varepsilon }$

The third bullet point means that the area under the graph of the function ${\displaystyle y=K_{n}(t)}$ becomes increasingly concentrated close to the origin as n approaches infinity. This definition lends us to the following theorem.

Theorem—The sequence of Landau kernels is a Dirac sequence

Proof: We prove the third property only. In order to do so, we introduce the following lemma:

Lemma—The coefficients satsify the following relationship, ${\displaystyle c_{n}\geq {\frac {2}{n+1}}}$

Proof of the Lemma:

Using the definition of the coefficients above, we find that the integrand is even, we may write$${\displaystyle {\frac {c_{n}}{2}}=\int _{0}^{1}(1-t^{2})^{n}\,dt=\int _{0}^{1}(1-t)^{n}(1+t)^{n}\,dt\geq \int _{0}^{1}(1-t)^{n}\,dt={\frac {1}{1+n}}}$$completing the proof of the lemma. A corollary of this lemma is the following:

Corollary—For all positive, real ${\displaystyle \delta$ :} ${\displaystyle \int _{\mathbb {R} \smallsetminus [-\delta ,\delta ]}K_{n}(t)\,dt\leq {\frac {2}{c_{n}}}\int _{\delta }^{1}(1-t^{2})^{n}\,dt\leq (n+1)(1-r^{2})^{n}}$

See also

• Poisson kernel
• Fejér kernel
• Dirichlet kernel