是不是有方法找general term ?

Of course we use Interpolation.

The most simple one of course is Lagrange Interpolating Polynomial:


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http://mathworld.wolfram.com/LagrangeInterpolatingPolynomial.html


For your question,



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你個example應該係冇general term，或者係一d好難想像的series。所以答唔到你。
如果係arithmetic Sequences，你可以將其中兩項相減得公差，d。咁 general term就係T(x)=首項+d(x-1)。
如果係geometic Sequences，你可以將其中兩項相除得公差，r。咁 general term就係T(x)=首項乘以r^(x-1)。
呢個世界上有好多唔同既series，好似質數咁都係一個series，但係你唔會搵到general term。又好似1,1,2,3,5,8.....，佢既 general term就係T(n)=T(n-2)+T(n-1)。呢d series 既general term係唔特別方法搵ga!要靠自己既想像力。

至於你話general term可不可以當function，其實general term就係一個function。係一個bijective function。即係function既每一個x都有一個y出現，而每一個y都可以搵番個x出現。
希望可以解答到你既問題。