前几天定义y1出现这样的报错，没管改成y11就继续写，今天比赛又忘记这个，赶紧上网查查是什么原因，这个y1在math.h里定义过了。继续打开math.h - corecrt_math.h
发现，不仅是y1,连j0,j1,jn,y0,yn都被定义了
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个贝塞尔函数什么什么的
参考：
Standard library header < math >
网上说有一个"gentlemen's agreement"，在库代码中暴露未记录的标识符会以下划线开头或结尾，用户代码不会，只是为了避免这类问题。
用来解决的build command：
cl foo.cpp /Feb /D _CRT_DECLARE_NONSTDC_NAMES=0
感觉这是历史遗留问题。。。。下次手动写y11