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A lowercase delta with two indices, one when they match and zero otherwise.
Also known as: Kronecker delta symbol, Delta ij, Discrete delta
The Kronecker delta is the identity matrix written as a symbol: one when its two indices are equal, zero when they are not. Under the summation convention it acts as an index-renaming device, which is what it is actually used for far more often than as a number.
Denotes the function equal to 1 when its two indices are equal and 0 otherwise — the entries of the identity matrix.
Contexts: Mathematics
Acts under summation as an index substitution: summing δ_ij against anything replaces one index with the other.
Contexts: Mathematics
Is discrete, unlike the Dirac delta it is named alongside; the two are related but are not the same object.
Contexts: Mathematics
A lowercase Greek delta, a small loop with an upward tail, carrying two small index letters.