add reference

Author: dkarrasch

src/kronecker.jl

@@ -84,7 +84,7 @@ end
 
 Construct a (lazy) representation of the Kronecker product `⨂ᵢ₌₁ⁿ Aᵢ` of at least 3 _square_
 Kronecker factors. In contrast to [`kron`](@ref), this function assumes that all Kronecker
-factors are square, and makes use of the following identity:
+factors are square, and makes use of the following identity[^1]:
 
 ```
 \\bigotimes_{i=1}^n A_i = \\prod_{i=1}^n I_1 \\otimes \\ldots \\otimes I_{i-1} \\otimes A_i \\otimes I_{i+1} \\otimes \\ldots \\otimes I_n
@@ -102,6 +102,10 @@ i.e., a `CompositeMap` where each factor is a Kronecker product consisting of th
 outer `UniformScalingMap`s and the respective Kronecker factor. This representation is
 expected to yield significantly faster multiplication (and reduce memory allocation)
 compared to [`kron`](@ref), but benchmarking intended use cases is highly recommended.
+
+[^1]: Fernandes, P. and Plateau, B. and Stewart, W. J. ["Efficient Descriptor-Vector
+Multiplications in Stochastic Automata Networks"](https://doi.org/10.1145/278298.278303),
+_Journal of the ACM_, 45(3), 381–414, 1998.
 """
 function squarekron(A::MapOrMatrix, B::MapOrMatrix, C::MapOrMatrix, Ds::MapOrMatrix...)
     maps = (A, B, C, Ds...)
@@ -328,7 +332,7 @@ kronsum(A::MapOrMatrix, B::MapOrMatrix, C::MapOrMatrix, Ds::MapOrMatrix...) =
 
 Construct a (lazy) representation of the Kronecker sum `A⊕B` of two or more square
 objects of type `LinearMap` or `AbstractMatrix`. This function makes use of the following
-representation of Kronecker sums:
+representation of Kronecker sums[^1]:
 
 ```
 \\bigoplus_{i=1}^n A_i = \\sum_{i=1}^n I_1 \\otimes \\ldots \\otimes I_{i-1} \\otimes A_i \\otimes I_{i+1} \\otimes \\ldots \\otimes I_n
@@ -364,6 +368,10 @@ julia> Δ₃ = D^⊕(2);
 julia> Matrix(Δ₁) == Matrix(Δ₂) == Matrix(Δ₃)
 true
 ```
+
+[^1]: Fernandes, P. and Plateau, B. and Stewart, W. J. ["Efficient Descriptor-Vector
+Multiplications in Stochastic Automata Networks"](https://doi.org/10.1145/278298.278303),
+_Journal of the ACM_, 45(3), 381–414, 1998.
 """
 function sumkronsum(A::MapOrMatrix, B::MapOrMatrix)
     LinearAlgebra.checksquare(A, B)