Added ref to hyperdualmatrixtools

README.md

@@ -33,6 +33,7 @@ Therefore, only the inverse of <a href="https://www.codecogs.com/eqnedit.php?lat
 This package makes available a `DualFactors` type which containts the factors of <a href="https://www.codecogs.com/eqnedit.php?latex=\fn_phv&space;A" target="_blank"><img src="https://latex.codecogs.com/gif.latex?\fn_phv&space;A" title="A" /></a> and the non-real parts of <a href="https://www.codecogs.com/eqnedit.php?latex=\fn_phv&space;M" target="_blank"><img src="https://latex.codecogs.com/gif.latex?\fn_phv&space;M" title="M" /></a>, and overloads `factorize` to create an instance of `DualFactors`, which can then be called with `\` to efficiently solve dual-valued linear systems of the type <a href="https://www.codecogs.com/eqnedit.php?latex=\fn_phv&space;M&space;x&space;=&space;b" target="_blank"><img src="https://latex.codecogs.com/gif.latex?\fn_phv&space;M&space;x&space;=&space;b" title="M x = b" /></a>. 
 
 This package should be useful for evaluation of second derivatives of functions that use `\` (e.g., with iterative solvers).
+Note the same idea extends to hyper dual numbers (see the [HyperDualMatrixTools.jl](https://github.com/briochemc/HyperDualMatrixTools.jl) package).
 
 ## Usage