前幾天定義y1出現這樣的報錯，沒管改成y11就繼續寫，今天比賽又忘記這個，趕緊上網查查是什么原因，這個y1在math.h里定義過了。繼續打開math.h - corecrt_math.h
發現，不僅是y1,連j0,j1,jn,y0,yn都被定義了
作用：
Function: double j0 (double x)
j0 returns the Bessel function of the first kind of order 0 of x. It may signal underflow if x is too large.
Function: double j1 (double x)
j1 returns the Bessel function of the first kind of order 1 of x. It may signal underflow if x is too large.
Function: double jn (int n, double x)
jn returns the Bessel function of the first kind of order n of x. It may signal underflow if x is too large.
Function: double y0 (double x)
y0 returns the Bessel function of the second kind of order 0 of x. It may signal underflow if x is too large. If x is negative, y0 signals a domain error; if it is zero, y0 signals overflow and returns -∞.
Function: double y1 (double x)y1 returns the Bessel function of the second kind of order 1 of x. It may signal underflow if x is too large. If x is negative, y1 signals a domain error; if it is zero, y1 signals overflow and returns -∞.
Function: double yn (int n, double x)yn returns the Bessel function of the second kind of order n of x. It may signal underflow if x is too large. If x is negative, yn signals a domain error; if it is zero, yn signals overflow and returns -∞.
大概意思就是返回一個貝塞爾函數什么什么的
參考：
Standard library header < math >
網上說有一個"gentlemen's agreement"，在庫代碼中暴露未記錄的標識符會以下划線開頭或結尾，用戶代碼不會，只是為了避免這類問題。
用來解決的build command：
cl foo.cpp /Feb /D _CRT_DECLARE_NONSTDC_NAMES=0
感覺這是歷史遺留問題。。。。下次手動寫y11