L=1
V0=1

V1x[x_]=(((4 V0)/π))  Sin[(( 2π)/L)1 x]

Plot[ V1x[x],{x,-1,1},PlotRange->{-1.3,1.3}]

V3x[x_]=(((4 V0)/π)) ( (1/3)Sin[(( 2π)/L)3 x])

Plot[ V3x[x],{x,-1,1},PlotRange->{-1.3,1.3}]

Plot[ V1x[x]+V3x[x],{x,-1,1}]

V1xz[x_,z_]=(((4 V0)/π))  Sin[(( 2π)/L)1 x] Exp[-(((2π)/L)1)z]

V3xz[x_,z_]=V3x[x_]=(((4 V0)/π)) ( (1/3)Sin[(( 2π)/L)3 x]) Exp[-(((2π)/L)3)z]

Plot3D[( V1xz[x,z]+V3xz[x,z]),{x,-1,1},{z,0,1},PlotRange->{-1.3,1.3}]

Plot3D[V3xz[x,z],{x,-1,1},{z,0,1},PlotRange->{-1.3,1.3}]

Plot3D[V1xz[x,z],{x,-1,1},{z,0,1},PlotRange->{-1.3,1.3}]

Unset[k]

Vnx[x_,n_]=Sum[(((4 V0)/π)) ( (1/k)Sin[(( 2π)/L)k x]),{k,1,n,2}];

Plot[ Vnx[x,3],{x,-1,1}]

Plot[ Vnx[x,5],{x,-1,1}]

Plot[ Vnx[x,11],{x,-1,1}]

Vnxz[x_,z_,n_]=Sum[(((4 V0)/π)) ( (1/k)Sin[(( 2π)/L)k x] Exp[-(( 2π)/L)k z]),{k,1,n,2}];

Plot3D[Vnxz[x,z,9],{x,-1,1},{z,0,1},PlotRange->{-1.3,1.3}]