Csdrhrt

How to draw these figures in TikZ?

I have managed to draw the first two figures, but the way I drew the arrows is not really good:

documentclass[tikz,border=3mm]{standalone} 

begin{document} 

begin{tikzpicture}

foreach i in {-3,...,3} 

    draw[->] (i,0)--(i,-1.1); % Figure A

   %draw[-<] (i,0)--(i,-1.1);   Figure B

draw (-3,-1)--(-3,-2.2);

draw (-2,-1)--(-2,-2.4);

draw (-1,-1)--(-1,-2.6);

draw (0,-1)--(0,-2.7);

draw (3,-1)--(3,-2.2);

draw (2,-1)--(2,-2.4);

draw (1,-1)--(1,-2.6);

% It is ugly and time-consuming, right? but I can't find a better one

draw[line width=3pt,red] (-3.5,0)--(3.5,0);

draw (0,0) node[above] {$+$};

draw[line width=3pt,blue] (-3.5,-2) to[out=-20,in=-160] (3.5,-2);

draw (0,-2.7) node[below] {$-$};

end{tikzpicture} 

end{document}

As the way I drew the straight arrows is not optimal, I can't use it to draw the curved arrows in the last two figures.

Can you show me a better way? Thank you in advance.

Welcome to TeX.SE! decoration.markings and foreach can help a lot here. The arrows in the middle are (essentially) from here. And decorations.markings is also used to set marks along the blue curves. They get used to define the starting points of the arrows. The to[out=...,int=...] is used to bend the paths in C and D.

documentclass[tikz,border=3mm]{standalone} 

usetikzlibrary{decorations.markings,arrows.meta}

tikzset{->-/.style={postaction={decorate,decoration={

  markings,

  mark=at position #1 with {arrow[arsty]{>}}}}},

  -<-/.style={postaction={decorate,decoration={

  markings,

  mark=at position #1 with {arrow[arsty]{<}}}}},

  insert target/.style args={#1|#2}{postaction={decorate,decoration={

  markings,

  mark=at position #1 with {coordinate (target-#2);}}}}

  }

begin{document} 

begin{tikzpicture}[arsty/.style={},>=Latex,font=sffamily]

 begin{scope}[local bounding box=A]

  draw[line width=3pt,red,

  %insert target/.list={1/12|Atop-1,3/12|Atop-2,5/12|Atop-3,7/12|Atop-4,9/12|Atop-5,11/12|Atop-6}

  ] (-3.5,0)--(3.5,0);

  draw (0,0) node[above] {$+$};

  draw[line width=3pt,blue,

  insert target/.list={1/12|Abot-1,3/12|Abot-2,5/12|Abot-3,7/12|Abot-4,9/12|Abot-5,11/12|Abot-6}] (-3.5,-2) to[out=-20,in=-160] (3.5,-2);

  foreach X in {1,...,6}

  {draw[-<-=0.5] (target-Abot-X) -- (0,0-|target-Abot-X);}

  draw (0,-2.7) node[below] {$-$};

 end{scope}

 node[anchor=south] at (A.north) {A};

 % 

 begin{scope}[local bounding box=B,xshift=8cm]

  draw[line width=3pt,red,

  %insert target/.list={1/12|Btop-1,3/12|Btop-2,5/12|Btop-3,7/12|Btop-4,9/12|Btop-5,11/12|Btop-6}

  ] (-3.5,0)--(3.5,0);

  draw (0,0) node[above] {$+$};

  draw[line width=3pt,blue,

  insert target/.list={1/12|Bbot-1,3/12|Bbot-2,5/12|Bbot-3,7/12|Bbot-4,9/12|Bbot-5,11/12|Bbot-6}] (-3.5,-2) to[out=-20,in=-160] (3.5,-2);

  foreach X in {1,...,6}

  {draw[->-=0.5] (target-Bbot-X) -- (0,0-|target-Bbot-X);}

  draw (0,-2.7) node[below] {$-$};

 end{scope} 

 node[anchor=south] at (B.north) {B};

 %

 begin{scope}[local bounding box=C,yshift=-6cm]

  draw[line width=3pt,red,

  %insert target/.list={1/12|Ctop-1,3/12|Ctop-2,5/12|Ctop-3,7/12|Ctop-4,9/12|Ctop-5,11/12|Ctop-6}

  ] (-3.5,0)--(3.5,0);

  draw (0,0) node[above] {$+$};

  draw[line width=3pt,blue,

  insert target/.list={1/12|Cbot-1,3/12|Cbot-2,5/12|Cbot-3,7/12|Cbot-4,9/12|Cbot-5,11/12|Cbot-6}] 

  (-3.5,-2) to[out=-20,in=-160] (3.5,-2);

  foreach X in {1,...,6}

  {draw[-<-=0.5] (target-Cbot-X) to[out=69+6*X,in=-90] 

  ([xshift={(3.5-X)*1.3mm}]target-Cbot-X |-0,0);

  }

  draw (0,-2.7) node[below] {$-$};

 end{scope}

 node[anchor=south] at (C.north) {C};

 % 

 begin{scope}[local bounding box=D,xshift=8cm,yshift=-6cm]

  draw[line width=3pt,red,

  insert target/.list={1/12|Dtop-1,3/12|Dtop-2,5/12|Dtop-3,7/12|Dtop-4,9/12|Dtop-5,11/12|Dtop-6}] (-3.5,0)--(3.5,0);

  draw (0,0) node[above] {$+$};

  draw[line width=3pt,blue,

  insert target/.list={1/12|Dbot-1,3/12|Dbot-2,5/12|Dbot-3,7/12|Dbot-4,9/12|Dbot-5,11/12|Dbot-6}] (-3.5,-2) to[out=-20,in=-160] (3.5,-2);

  foreach X in {1,...,6}

  {draw[->-=0.5] (target-Dbot-X) to[out=69+6*X,in=-90] 

  ([xshift={(3.5-X)*1.3mm}]target-Dbot-X |-0,0);

  }

  draw (0,-2.7) node[below] {$-$};

 end{scope} 

 node[anchor=south] at (D.north) {D};

end{tikzpicture} 

end{document}

An alternative, but very similar, approach to marmot's excellent solution (beaten by mere minutes).

documentclass[tikz,border=3mm]{standalone}

begin{document}

begin{tikzpicture}

% Library for middle arrowheads

usetikzlibrary{decorations.markings}



    % Style the vertical lines (this form is necessary for the middle arrowhead)

    begin{scope}[semithick,decoration={

        markings,

        mark=at position 0.4 with {arrow{>}}}

    ]

    % Loop through and draw each vertical line

    foreach xRed in {-3,...,3}

        % Spread the bottom points horizontally

        pgfmathsetmacroxBlue{xRed*1.1}

        % Red line is straight, but could put arbitrary function here

        pgfmathsetmacroyRed{0}

        % Blue line is curved, quadratic eqn

        pgfmathsetmacroyBlue{0.06*xBlue*xBlue-2.7}

        % Calculate angle of lines touching blue line

        pgfmathsetmacroangle{6.6667*xRed+90}

        % Draw a line

        draw[postaction={decorate}] (xRed,yRed) to[out=-90,in=angle] (xBlue,yBlue);

    end{scope}



    % Red line, could put arbitrary function here

    draw[domain=-3.5:3.5,smooth,variable=x,line width=3pt,red] plot ({x},{0});

    % + sign

    draw (0,0) node[above] {(+)};



    % Blue line, quadratic eqn

    draw[domain=-3.5:3.5,smooth,variable=x,line width=3pt,blue] plot ({x},{0.06*x*x-2.7});

    % - sign

    draw (0,-2.7) node[below] {(-)};



end{tikzpicture}

end{document}