code: public static boolean isPositiveDefinite( DMatrixRMaj A ) {
          if( !isSquare(A))
             return false;
  
          CholeskyDecompositionInner_DDRM chol = new CholeskyDecompositionInner_DDRM(true);
          if( chol.inputModified() )
              A = A.copy();
  
          return chol.decompose(A);
      }
docs: <p>
  Checks to see if the matrix is positive definite.
  </p>
  <p>
  x<sup>T</sup> A x &gt; 0<br>
  for all x where x is a non-zero vector and A is a symmetric matrix.
  </p>
  
  @param A square symmetric matrix. Not modified.
  
  @return True if it is positive definite and false if it is not.