以下函数的绘图是正确的，但是当评估负值时，它们与绘图不对应
\documentclass[border=1pt]{standalone}
\usepackage[dvipsnames,svgnames,x11names,]{xcolor}
\usepackage{pgf,tikz,tikz-3dplot}
\usepackage{pgfplots}
\pgfplotsset{compat=1.12}
\begin{document}
\pgfmathsetmacro{\xi}{-2.5} %value
\begin{tikzpicture}[line cap=round,line join=round, font={\small }]
\begin{axis}[height=6cm,width=10cm,no markers, axis lines=center, xlabel={$x$},
ylabel={$F(x)$},
xmin=-9,xmax=9,
ymin=-7,ymax=7,
declare function={
erf(\x)=%
(1+(e^(-(\x*\x))*(-265.057+abs(\x)*(-135.065+abs(\x)%
*(-59.646+(-6.84727-0.777889*abs(\x))*abs(\x)))))%
/(3.05259+abs(\x))^5)*(\x>0?1:-1);
f(\x)=(-0.5*\x^3+3.8*\x^2+4*\x+1)*exp(-0.6*\x^2);
Fa(\x)=0.5*exp(-0.6*\x^2)*(-(-0.5)*0.6*\x^2-3.8*0.6*\x-4*0.6+0.5)/(0.6^2);
Fb(\x)=-0.25*sqrt(pi)*(2*0.6+3.8)*erf(-sqrt(0.6)*\x)/(0.6*sqrt(0.6));
F(\x)=Fa(\x)+Fb(\x);
},
]
\addplot[domain=-5:5, samples=41, smooth, SteelBlue1, line width=1pt]{F(x)};
\draw[dash pattern=on1pt off 1pt, ] (\xi,0)-- (\xi,{F(\xi)});
\draw[gray] (\xi,0.2) -- (\xi, -0.2);
\draw[, DodgerBlue3] (0,{F(\xi)})-- (\xi,{F(\xi)});
\node[font=\tiny] at (4,-5) {$F(a)=\pgfmathparse{F(\xi)}\pgfmathresult$};
\end{axis}
\end{tikzpicture}
\end{document}
我正在使用这个 erf 函数LaTeX 中的 Erf 函数
答案1
在您的函数中替换\x^2为(\x)^2：
\documentclass[border=1pt]{standalone}
\usepackage[dvipsnames,svgnames,x11names,]{xcolor}
%\usepackage{pgf,tikz,tikz-3dplot}
\usepackage{pgfplots}
\pgfplotsset{compat=1.12} % recent is 1.17, I suggest you to upgrade it
\begin{document}
\pgfmathsetmacro{\xi}{-2.5} %value
\begin{tikzpicture}[line cap=round,line join=round, font={\small }]
\begin{axis}[height=6cm,width=10cm,no markers, axis lines=center, xlabel={$x$},
ylabel={$F(x)$},
xmin=-9,xmax=9,
ymin=-7,ymax=7,
declare function={
erf(\x)=%
(1+(e^(-(\x*\x))*(-265.057+abs(\x)*(-135.065+abs(\x)%
*(-59.646+(-6.84727-0.777889*abs(\x))*abs(\x)))))%
/(3.05259+abs(\x))^5)*(\x>0?1:-1);
f(\x)=(-0.5*\x^3+3.8*\x^2+4*\x+1)*exp(-0.6*\x^2);
Fa(\x)=0.5*exp(-0.6*(\x)^2)*(-(-0.5)*0.6*(\x)^2-3.8*0.6*\x-4*0.6+0.5)/(0.6^2);
Fb(\x)=-0.25*sqrt(pi)*(2*0.6+3.8)*erf(-sqrt(0.6)*\x)/(0.6*sqrt(0.6));
F(\x)=Fa(\x)+Fb(\x);
},
]
\addplot[domain=-5:5, samples=41, smooth, SteelBlue1, line width=1pt]{F(x)};
\draw[dash pattern=on1pt off 1pt, ] (\xi,0)-- (\xi,{F(\xi)});
\draw[gray] (\xi,0.2) -- (\xi, -0.2);
\draw[, DodgerBlue3] (0,{F(\xi)})-- (\xi,{F(\xi)});
\node[font=\tiny] at (4,-5) {$F(a)=\pgfmathparse{F(\xi)}\pgfmathresult$};
\end{axis}
\end{tikzpicture}
\end{document}
