Kronecker delta

Named after German mathematician Leopold Kronecker (1823–1891)

Kronecker delta (plural Kronecker deltas)

1. (mathematics) A binary function, written as δ with two subscripts, which evaluates to 1 when its arguments are equal, and 0 otherwise.
   • 1998, Robert G. Deissler, Turbulent Fluid Motion, Taylor & Francis, page 24,

     The Kronecker delta ${\displaystyle \delta _{ij}}$ is an example of an isotropic tensor. That is, its components remain invariant with rotation of coordinate axes.

   • 2007, J. N. Reddy, An Introduction to Continuum Mechanics, Cambridge University Press, page 20,

     Further, the Kronecker delta and the permutation symbol are related by the identity, known as the ${\displaystyle e}$ -${\displaystyle \delta }$ identity [see Problem 2.5(d)],

     ${\displaystyle e_{ijk}e_{imn}=\delta _{jm}\delta _{kn}-\delta _{jn}\delta _{km}}$ .      (2.2.43)

     The permutation symbol and the Kronecker delta prove to be very useful in proving vector identities.

   • 2017, Sadri Hassani, Special Relativity: A Heuristic Approach, Elsevier, page 257,

     The most important property of the Kronecker delta occurs when it shares a common repeated index with another tensor:

     Note 10.1.3. When an index of a tensor ${\displaystyle T}$ is contracted with one of the indices of the Kronecker delta, the result is an expression in which the Kronecker delta is removed and the the contracted index of ${\displaystyle T}$ is replaced by the other index of the Kronecker delta.

2. (mathematics) A unary function, written as δ with a single subscript, which evaluates to 1 at zero, and 0 elsewhere.

• The notation ${\displaystyle \delta ^{ij}}$ and ${\displaystyle \delta _{j}^{i}}$ are also sometimes used.
• In linear algebra, the Kronecker delta can be regarded as a tensor of type (1,1).
• The function can also be expressed using Iverson bracket notation, as ${\displaystyle [i=j]}$ .
• The single-argument function is equivalent to setting ${\displaystyle j=0}$ in the binary function.

• (binary function): Kronecker tensor, substitution tensor

• generalized Kronecker delta

• Dirac delta
• Dirac delta function
• Dirac measure
• Iverson bracket
• Kronecker product
• Kronecker symbol
• Levi-Civita symbol

• Unit function on Wikipedia.Wikipedia
• Iverson bracket on Wikipedia.Wikipedia