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+(UNDER CONSTRUCTION)
 This library provides a set of functions for computation derivatives using spectral methods. The lib depends on [FFTW](http://www.fftw.org/) library. The library is compatible with W-SLDA Toolkit however, it can be used as standalone lib. It is written in [C99 standard](https://en.wikipedia.org/wiki/C99).
 # Name convention
 Generic name of a function is `wderiv_operation_Nd_t`, where: 
@@ -24,17 +25,79 @@ Generic name of a function is `wderiv_operation_Nd_t`, where:
 * `t`:
   * `r` for real problems
   * `c` for complex problems
-# Sample declarations
+# Function Documentation
+## 3D derivatives:
+### Basic 3D derivative function:
 ```c
-/**
- * Function computes first derivative with respect to x (df/dx)
- * of 3D real function f(x,y,z)   
- * @param f pointer to function, array of size [nx*ny*nz] (INPUT)
- * @param dfdx pointer to function, array of size [nx*ny*nz] (OUTPUT)
- *                NOTE: dfdx can be the same pointer as f, then result overwrites input 
- * @return error code
- * */
-int wderiv_dfdx_3d_r(double *f, double *dfdx);
+int wderiv_derivative_3d_r(int direction, int n, double *f, double *deriv)
+```
+Function computes n-th derivative with respect to `direction` of 3D real function $`f(x,y,z)`$: $`\frac{\partial^n f}{\partial t^n}`$.   
+ * `direction` - predefined integer corresponding to direction of derivative: `WDERIV_DX` or `WDERIV_DY` or `WDERIV_DZ`
+ * `f` - pointer to input function which is array of size [nx\*ny\*nz]
+ * `deriv` - pointer to output derivative function which is array of size [nx\*ny\*nz]
+ * `return` - error code
+
+`deriv` can be the same pointer as `f`, then result overwrites input. 
+### First 3D derivative functions:
+```c
+int wderiv_dfdx_3d_r(double *f, double *dfdx)
+```
+Function computes first derivative with respect to $`x`$ of 3D real function $`f(x,y,z)`$: $`\frac{\partial f}{\partial x}`$.   
+ * `f` - pointer to input function which is array of size [nx\*ny\*nz]
+ * `dfdx` - pointer to output derivative function which is array of size [nx\*ny\*nz]
+ * `return` - error code
+
+`dfdx` can be the same pointer as `f`, then result overwrites input. 
+ 
+```c
+int wderiv_dfdy_3d_r(double *f, double *dfdy)
+```
+Function computes first derivative with respect to $`y`$ of 3D real function $`f(x,y,z)`$: $`\frac{\partial f}{\partial y}`$.   
+ * `f` - pointer to input function which is array of size [nx*ny*nz]
+ * `dfdy` - pointer to output derivative function which is array of size [nx*ny*nz]
+ * `return` - error code
+
+`dfdy` can be the same pointer as `f`, then result overwrites input. 
+ 
+```c
+int wderiv_dfdz_3d_r(double *f, double *dfdz)
+```
+Function computes first derivative with respect to $`z`$ of 3D real function $`f(x,y,z)`$: $`\frac{\partial f}{\partial x}`$.   
+ * `f` - pointer to input function which is array of size [nx*ny*nz]
+ * `dfdz` - pointer to output derivative function which is array of size [nx*ny*nz]
+ * `return` - error code
+
+`dfdz` can be the same pointer as `f`, then result overwrites input. 
+### Second 3D derivative functions:
+```c
+int wderiv_d2fdx2_3d_r(double *f, double *d2fdx2)
+```
+Function computes second derivative with respect to $`x`$ of 3D real function $`f(x,y,z)`$: $`\frac{\partial^2 f}{\partial x^2}`$.   
+ * `f` - pointer to input function which is array of size [nx\*ny\*nz]
+ * `d2fdx2` - pointer to output derivative function which is array of size [nx\*ny\*nz]
+ * `return` - error code
+
+`d2fdx2` can be the same pointer as `f`, then result overwrites input. 
+ 
+```c
+int wderiv_d2fdy2_3d_r(double *f, double *d2fdy2)
+```
+Function computes second derivative with respect to $`y`$ of 3D real function $`f(x,y,z)`$: $`\frac{\partial^2 f}{\partial y^2}`$.   
+ * `f` - pointer to input function which is array of size [nx\*ny\*nz]
+ * `d2fdy2` - pointer to output derivative function which is array of size [nx\*ny\*nz]
+ * `return` - error code
+
+`d2fdy2` can be the same pointer as `f`, then result overwrites input. 
+ 
+```c
+int wderiv_d2fdz2_3d_r(double *f, double *d2fdz2)
+```
+Function computes second derivative with respect to $`z`$ of 3D real function $`f(x,y,z)`$: $`(\frac{d^2f}{dz^2})`$.   
+ * `f` - pointer to input function which is array of size [nx*ny*nz]
+ * `d2fdz2` - pointer to output derivative function which is array of size [nx*ny*nz]
+ * `return` - error code
+
+`d2fdz2` can be the same pointer as `f`, then result overwrites input. 
 
 /**
  * Function computes gradient