[doc] Split linear solver article from sparse_matrix.md.

docs/lang/articles/math/linear_solver.md

@@ -0,0 +1,73 @@
+---
+sidebar_position: 3
+---
+
+# Linear Solver
+
+Solving linear equations is a common task in scientific computing. Taichi provides basic direct and iterative linear solvers for
+various simulation scenarios. Currently, there are two categories of linear solvers available:
+1. Solvers built for `SparseMatrix`
+2. Solvers built for `ti.field`
+
+## Sparse linear solver
+You may want to solve some linear equations using sparse matrices.
+Then, the following steps could help:
+1. Create a `solver` using `ti.linalg.SparseSolver(solver_type, ordering)`. Currently, the factorization types supported on CPU backends are `LLT`, `LDLT`, and `LU`, and supported orderings include `AMD` and `COLAMD`. The sparse solver on CUDA supports the `LLT` factorization type only.
+2. Analyze and factorize the sparse matrix you want to solve using `solver.analyze_pattern(sparse_matrix)` and `solver.factorize(sparse_matrix)`
+3. Call `x = solver.solve(b)`, where `x` is the solution and `b` is the right-hand side of the linear system. On CPU backends, `x` and `b` can be NumPy arrays, Taichi Ndarrays, or Taichi fields. On the CUDA backend, `x` and `b` *must* be Taichi Ndarrays.
+4. Call `solver.info()` to check if the solving process succeeds.
+
+Here's a full example.
+
+```python
+import taichi as ti
+
+arch = ti.cpu # or ti.cuda
+ti.init(arch=arch)
+
+n = 4
+
+K = ti.linalg.SparseMatrixBuilder(n, n, max_num_triplets=100)
+b = ti.ndarray(ti.f32, shape=n)
+
+@ti.kernel
+def fill(A: ti.types.sparse_matrix_builder(), b: ti.template(), interval: ti.i32):
+    for i in range(n):
+        A[i, i] += 2.0
+
+        if i % interval == 0:
+            b[i] += 1.0
+
+fill(K, b, 3)
+
+A = K.build()
+print(">>>> Matrix A:")
+print(A)
+print(">>>> Vector b:")
+print(b)
+# outputs:
+# >>>> Matrix A:
+# [2, 0, 0, 0]
+# [0, 2, 0, 0]
+# [0, 0, 2, 0]
+# [0, 0, 0, 2]
+# >>>> Vector b:
+# [1. 0. 0. 1.]
+solver = ti.linalg.SparseSolver(solver_type="LLT")
+solver.analyze_pattern(A)
+solver.factorize(A)
+x = solver.solve(b)
+isSuccess = solver.info()
+print(">>>> Solve sparse linear systems Ax = b with the solution x:")
+print(x)
+print(f">>>> Computation was successful?: {isSuccess}")
+# outputs:
+# >>>> Solve sparse linear systems Ax = b with the solution x:
+# [0.5 0.  0.  0.5]
+# >>>> Computation was successful?: True
+```
+## Examples
+
+Please have a look at our two demos for more information:
++ [Stable fluid](https://github.com/taichi-dev/taichi/blob/master/python/taichi/examples/simulation/stable_fluid.py): A 2D fluid simulation using a sparse Laplacian matrix to solve Poisson's pressure equation.
++ [Implicit mass spring](https://github.com/taichi-dev/taichi/blob/master/python/taichi/examples/simulation/implicit_mass_spring.py): A 2D cloth simulation demo using sparse matrices to solve the linear systems.

docs/lang/articles/math/sparse_matrix.md

@@ -131,66 +131,3 @@ print(f">>>> Element Access: A[0,0] = {A[0,0]}")
 # outputs:
 # >>>> Element Access: A[0,0] = 1.0
 ```
-
-## Sparse linear solver
-You may want to solve some linear equations using sparse matrices.
-Then, the following steps could help:
-1. Create a `solver` using `ti.linalg.SparseSolver(solver_type, ordering)`. Currently, the factorization types supported on CPU backends are `LLT`, `LDLT`, and `LU`, and supported orderings include `AMD` and `COLAMD`. The sparse solver on CUDA supports the `LLT` factorization type only.
-2. Analyze and factorize the sparse matrix you want to solve using `solver.analyze_pattern(sparse_matrix)` and `solver.factorize(sparse_matrix)`
-3. Call `x = solver.solve(b)`, where `x` is the solution and `b` is the right-hand side of the linear system. On CPU backends, `x` and `b` can be NumPy arrays, Taichi Ndarrays, or Taichi fields. On the CUDA backend, `x` and `b` *must* be Taichi Ndarrays.
-4. Call `solver.info()` to check if the solving process succeeds.
-
-Here's a full example.
-
-```python
-import taichi as ti
-
-arch = ti.cpu # or ti.cuda
-ti.init(arch=arch)
-
-n = 4
-
-K = ti.linalg.SparseMatrixBuilder(n, n, max_num_triplets=100)
-b = ti.ndarray(ti.f32, shape=n)
-
-@ti.kernel
-def fill(A: ti.types.sparse_matrix_builder(), b: ti.template(), interval: ti.i32):
-    for i in range(n):
-        A[i, i] += 2.0
-
-        if i % interval == 0:
-            b[i] += 1.0
-
-fill(K, b, 3)
-
-A = K.build()
-print(">>>> Matrix A:")
-print(A)
-print(">>>> Vector b:")
-print(b)
-# outputs:
-# >>>> Matrix A:
-# [2, 0, 0, 0]
-# [0, 2, 0, 0]
-# [0, 0, 2, 0]
-# [0, 0, 0, 2]
-# >>>> Vector b:
-# [1. 0. 0. 1.]
-solver = ti.linalg.SparseSolver(solver_type="LLT")
-solver.analyze_pattern(A)
-solver.factorize(A)
-x = solver.solve(b)
-isSuccess = solver.info()
-print(">>>> Solve sparse linear systems Ax = b with the solution x:")
-print(x)
-print(f">>>> Computation was successful?: {isSuccess}")
-# outputs:
-# >>>> Solve sparse linear systems Ax = b with the solution x:
-# [0.5 0.  0.  0.5]
-# >>>> Computation was successful?: True
-```
-## Examples
-
-Please have a look at our two demos for more information:
-+ [Stable fluid](https://github.com/taichi-dev/taichi/blob/master/python/taichi/examples/simulation/stable_fluid.py): A 2D fluid simulation using a sparse Laplacian matrix to solve Poisson's pressure equation.
-+ [Implicit mass spring](https://github.com/taichi-dev/taichi/blob/master/python/taichi/examples/simulation/implicit_mass_spring.py): A 2D cloth simulation demo using sparse matrices to solve the linear systems.