import sympy


def _sympy_tridiag(a, b):
    """Creates the tridiagonal sympy matrix tridiag(b, a, b).
    """
    n = len(a)
    assert n == len(b)
    A = [[0 for _ in range(n)] for _ in range(n)]
    for i in range(n):
        A[i][i] = a[i]
    for i in range(n - 1):
        A[i][i + 1] = b[i + 1]
        A[i + 1][i] = b[i + 1]
    return sympy.Matrix(A)