Pinch theorem is a very important method in the limit of sequence, and it is also easy to work out comprehensive problems. The core of the pinch theorem is how to scale the sequence reasonably, which is also the difficulty in the use of the pinch theorem. Pinch theorem is generally used for the limit of n terms and expressions, and the function is not continuous.

To put it simply, it is known that your eldest brother and your third brother were born on the same day, and you three are triplets, which can prove that you were born on the same day.

What changes is the term behind the square of n, and the law is from 1 to n, the largest is n, and the smallest is 1. If all items are changed to 1, zoom in; if all items are changed to n, zoom out, and the answer can be deduced immediately.

Pinching criterion is that for three functions a (x), b(x) and c (x), if a (x) < b (x) < c (x) is established in the neighborhood of a certain point x0, when X tends to x0, the limit values of a(x) and c(x) are equal (we set this limit value as m).