函数<img src="https://up.zaixiankaoshi.com/docs/c9/1e/c91ec7e14ac75a3451a0c01f26722b7a.png">的频谱函数为<img src="https://up.zaixiankaoshi.com/docs/58/8b/588b849defc09062ae82aa14b29153ff.png">，则函数<img src="https://up.zaixiankaoshi.com/docs/ea/0b/ea0bb67ee07c44a2ecb80d7888f6928f.png">的频谱函数为（ ）