/**
 * Checks if a given matrix is diagonal.
 *
 * A matrix is considered diagonal if it is square (i.e., the number of rows equals the number of columns)
 * and all elements outside the main diagonal are zero.
 *
 * @param matrix - A two-dimensional array of numbers representing the matrix to check.
 * @returns `true` if the matrix is diagonal, otherwise `false`.
 */
export declare function isDiagonal(matrix: number[][]): boolean;