class_docstr: Construct a stiffness matrix in various formats of Mikota pair.
  
  The stiffness matrix `K` is square real tri-diagonal symmetric
  positive definite with integer entries. 
  
  Parameters
  ----------
  shape : tuple of int
      The shape of the matrix.
  dtype : dtype
      Numerical type of the array. Default is ``np.int32``.
  
  Methods
  -------
  toarray()
      Construct a dense array from Mikota data
  tosparse()
      Construct a sparse array from Mikota data
  tobanded()
      The format for banded symmetric matrices,
      i.e., (2, n) ndarray with 2 upper diagonals
      placing the main diagonal at the bottom.
docstr: Construct matrix-free callable matrix-matrix multiplication by
  the Stiffness mass matrix without constructing or storing the matrix itself
  by reusing the ``_matvec(x)`` that supports both 1D and 2D arrays ``x``.
docstr_tok: ["Construct","matrix-free","callable","matrix-matrix","multiplication","by","the","Stiffness","mass","matrix","without","constructing","or","storing","the","matrix","itself","by","reusing","the","``","_matvec","(","x",")","``","that","supports","both","1D","and","2D","arrays","``","x","``","."]
code: def _matmat(self, x):
      """
      Construct matrix-free callable matrix-matrix multiplication by
      the Stiffness mass matrix without constructing or storing the matrix itself
      by reusing the ``_matvec(x)`` that supports both 1D and 2D arrays ``x``.
      """
      return self._matvec(x)
  
imports: import numpy
  from scipy.sparse.linalg import LinearOperator
  from scipy.sparse import kron, eye, dia_array
label_desc: Use image node_id 6 for calling the MikotaK obj's underlying member method code with example usage: obj._matmat(x) and returns: self