对于域k上多元多项式环k[x1,…,xn]中不可约单项式理想I、J、K和L，证明reg（IJKL）≤reg（I）+reg（J）+reg（K）+reg（L）.
Let I, J, K, and L be irreducible monomial ideals in a polynomial ring over a field k. In this paper, we prove reg(IJKL) ≤ reg(I)+reg(J)+reg(K)+reg(L).
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