Added documentation for symmetric and hermitian matrices.

Author: loiseaujc

doc/specs/stdlib_specialmatrices.md

@@ -12,7 +12,8 @@ The `stdlib_specialmatrices` module provides derived types and specialized drive
 These include:
 
 - Tridiagonal matrices
-- Symmetric Tridiagonal matrices (not yet supported)
+- Symmetric Tridiagonal matrices
+- Hermitian Tridiagonal matrices
 - Circulant matrices (not yet supported)
 - Toeplitz matrices (not yet supported)
 - Hankel matrices (not yet supported)
@@ -76,6 +77,107 @@ Tridiagonal matrices are available with all supported data types as `tridiagonal
 {!example/specialmatrices/example_tridiagonal_dp_type.f90!}
 ```
 
+### Symmetric tridiagonal matrices {#SymTridiagonal}
+
+#### Status
+
+Experimental
+
+#### Description
+
+Symmetric tridiagonal matrices are ubiquituous in scientific computing and often appear when discretizing 1D differential operators.
+A generic symmetric tridiagonal matrix has the following structure:
+$$
+    A
+    =
+    \begin{bmatrix}
+        a_1 &  b_1  \\
+        b_1 &  a_2      &  b_2  \\
+            &  \ddots   &  \ddots   &  \ddots   \\
+            &           &  b_{n-2} &  a_{n-1}  &  b_{n-1} \\
+            &           &           &  b_{n-1} &  a_n
+    \end{bmatrix}.
+$$
+Hence, only one vector of size `n` and two of size `n-1` need to be stored to fully represent the matrix.
+This particular structure also lends itself to specialized implementations for many linear algebra tasks.
+Interfaces to the most common ones will soon be provided by `stdlib_specialmatrices`.
+Symmetric tridiagonal matrices are available with all supported data types as `symtridiagonal_<kind>_type`, for example:
+
+- `symtridiagonal_sp_type`   : Symmetric tridiagonal matrix of size `n` with `real`/`single precision` data.
+- `symtridiagonal_dp_type`   : Symmetric tridiagonal matrix of size `n` with `real`/`double precision` data.
+- `symtridiagonal_xdp_type`  : Symmetric tridiagonal matrix of size `n` with `real`/`extended precision` data.
+- `symtridiagonal_qp_type`   : Symmetric tridiagonal matrix of size `n` with `real`/`quadruple precision` data.
+- `symtridiagonal_csp_type`  : Symmetric tridiagonal matrix of size `n` with `complex`/`single precision` data.
+- `symtridiagonal_cdp_type`  : Symmetric tridiagonal matrix of size `n` with `complex`/`double precision` data.
+- `symtridiagonal_cxdp_type` : Symmetric tridiagonal matrix of size `n` with `complex`/`extended precision` data.
+- `symtridiagonal_cqp_type`  : Symmetric tridiagonal matrix of size `n` with `complex`/`quadruple precision` data.
+
+#### Syntax
+
+- To construct a symmetric tridiagonal matrix from already allocated arrays `dv` (main diagonal, size `n`) and `ev` (upper diagonal, size `n-1`):
+
+`A = ` [[stdlib_specialmatrices(module):symtridiagonal(interface)]] `(dv, ev)`
+
+- To construct a symmetric tridiagonal matrix of size `n x n` with constant diagonal elements `dv`, and `ev`:
+
+`A = ` [[stdlib_specialmatrices(module):symtridiagonal(interface)]] `(dv, ev, n)`
+
+#### Example
+
+```fortran
+{!example/specialmatrices/example_symtridiagonal_dp_type.f90!}
+```
+
+### Hermitian tridiagonal matrices {#HermTridiagonal}
+
+#### Status
+
+Experimental
+
+#### Description
+
+Hermitian tridiagonal matrices are ubiquituous in scientific computing.
+A generic hermitian tridiagonal matrix has the following structure:
+$$
+    A
+    =
+    \begin{bmatrix}
+        a_1 &  b_1  \\
+        \bar{b}_1 &  a_2      &  b_2  \\
+            &  \ddots   &  \ddots   &  \ddots   \\
+            &           &  \bar{b}_{n-2} &  a_{n-1}  &  b_{n-1} \\
+            &           &           &  \bar{b}_{n-1} &  a_n
+    \end{bmatrix},
+$$
+where \( a_i \in \mathbb{R} \), \( b_i \in \mathbb{C} \) and the overbar denotes the complex conjugate operation.
+Hence, only one vector of size `n` and two of size `n-1` need to be stored to fully represent the matrix.
+This particular structure also lends itself to specialized implementations for many linear algebra tasks.
+Interfaces to the most common ones will soon be provided by `stdlib_specialmatrices`.
+Hermitian tridiagonal matrices are available with all supported data types as `hermtridiagonal_<kind>_type`, for example:
+
+- `hermtridiagonal_csp_type`  : Hermitian tridiagonal matrix of size `n` with `complex`/`single precision` data.
+- `hermtridiagonal_cdp_type`  : Hermitian tridiagonal matrix of size `n` with `complex`/`double precision` data.
+- `hermtridiagonal_cxdp_type` : Hermitian tridiagonal matrix of size `n` with `complex`/`extended precision` data.
+- `hermtridiagonal_cqp_type`  : Hermitian tridiagonal matrix of size `n` with `complex`/`quadruple precision` data.
+
+#### Syntax
+
+- To construct a hermitian tridiagonal matrix from already allocated arrays `dv` (main diagonal, size `n`) and `ev` (upper diagonal, size `n-1`):
+
+`A = ` [[stdlib_specialmatrices(module):hermtridiagonal(interface)]] `(dv, ev)`
+
+- To construct a hermitian tridiagonal matrix of size `n x n` with constant diagonal elements `dv`, and `ev`:
+
+`A = ` [[stdlib_specialmatrices(module):hermtridiagonal(interface)]] `(dv, ev, n)`
+
+Note that only the real parts of the diagonal elements `dv` are being used to construct the corresponding Hermitian matrix.
+
+#### Example
+
+```fortran
+{!example/specialmatrices/example_hermtridiagonal_dp_type.f90!}
+```
+
 ## Specialized drivers for linear algebra tasks
 
 Below is a list of all the specialized drivers for linear algebra tasks currently provided by the `stdlib_specialmatrices` module.
@@ -111,7 +213,7 @@ With the exception of `extended precision` and `quadruple precision`, all the ty
 - `op` (optional) : In-place operator identifier. Shall be a character(1) argument. It can have any of the following values: `N`: no transpose, `T`: transpose, `H`: hermitian or complex transpose.
 
 @warning
-Due to limitations of the underlying `lapack` driver, currently `alpha` and `beta` can only take one of the values `[-1, 0, 1]` for `tridiagonal` and `symtridiagonal` matrices. See `lagtm` for more details.
+Due to limitations of the underlying `lapack` driver, currently `alpha` and `beta` can only take one of the values `[-1, 0, 1]` for `tridiagonal`, `symtridiagonal` and `hermtridiagonal` matrices. See `lagtm` for more details.
 @endwarning
 
 #### Examples
@@ -192,17 +294,17 @@ Experimental
 
 The definition of all standard artihmetic operators have been overloaded to be applicable for the matrix types defined by `stdlib_specialmatrices`:
 
-- Overloading the `+` operator for adding two matrices of the same type and kind.
-- Overloading the `-` operator for subtracting two matrices of the same type and kind.
+- Overloading the `+` operator for adding two matrices of the same class and kind.
+- Overloading the `-` operator for subtracting two matrices of the same class and kind.
 - Overloading the `*` for scalar-matrix multiplication.
 
 #### Syntax
 
-- Adding two matrices of the same type:
+- Adding two matrices of the same class:
 
 `C = A` [[stdlib_specialmatrices(module):operator(+)(interface)]] `B`
 
-- Subtracting two matrices of the same type:
+- Subtracting two matrices of the same class:
 
 `C = A` [[stdlib_specialmatrices(module):operator(-)(interface)]] `B`
 
@@ -211,5 +313,5 @@ The definition of all standard artihmetic operators have been overloaded to be a
 `B = alpha` [[stdlib_specialmatrices(module):operator(*)(interface)]] `A`
 
 @note
-For addition (`+`) and subtraction (`-`), matrices `A`, `B` and `C` all need to be of the same type and kind. For scalar multiplication (`*`), `A` and `B` need to be of the same type and kind, while `alpha` is either `real` or `complex` (with the same kind again) depending on the type being used.
+For addition (`+`) and subtraction (`-`), matrices `A`, and `B` need to be of the same class and kind. For scalar multiplication (`*`), `A` and `B` need to be of the same class and kind, while `alpha` is either `real` or `complex` (with the same kind again) depending on the type being used.
 @endnote