众所周知，在古典逻辑中，逆否对称性是永真公式。

As we know, contrapositive symmetry is the tautology in classical logic.

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此外，本系统中的各组原始公理还是自足的，即如想推出只含某些函数符号的永真公式时只须使用相应的公理便成。

This system is self-suffient in the sense that if we wish to deduce an equality involving only some of the three functions, then we need only to make use of the corresponding axioms.

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研究了模糊模态逻辑中的永真式与基本模态逻辑中的有效公式之间的关系。

Relationship between tautologies in fuzzy modal logic and valid formulas in basic modal logic is investigated.

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