This paper analyzes on the reasons why the accidents of the gas explosion occur in the tunnel,and puts forward some methods,measures and matters needing attention of the safe discharge of the gas.

This article relates to the process of n-order matrix, the full permutation of n×n elements which is implemented by C++ program language, and resolves the problem of which traditional programming method can not implement the full permutation of all elements in that matrix.

The author infers from of the recurrence the formulas of full permutation,selected permutation, repeaed permutiation, combination,repeated combination,and solves problems in permutation and combination through examples.

Complete Ranking of Decision-making Units with Interval Data under Different Decision Environments;