3. Function graph as a manifold¶

This notebook is part of the Introduction to manifolds in SageMath by Andrzej Chrzeszczyk (Jan Kochanowski University of Kielce, Poland).

In [1]:

version()

Out[1]:

'SageMath version 9.6, Release Date: 2022-05-15'

The graph $$Γ(f)=\{(x, f(x))\in R^{m+n}: x ∈ U \},$$

for a smooth function $f : U → R^{m}$ and an open subset $U\subset R^n$ is a simple example of a smooth manifold having an atlas with a single chart

$$(Γ( f ), φ ),\quad \phi: (x, f (x)) \to x,\quad x\in U.$$

One-dimensional function graph as a manifold¶

Since we don't use any predefined charts or transitions in this case, we can use general manifold (not the Euclidean space as previously).

Example 3.1

Here we use an open interval $(0,2\pi)$:

In [2]:

J = manifolds.OpenInterval(0, 2*pi)  # open interval J as manifold
c.<t>=J.chart()                      # chart on J

First we plot $U=(0,2\pi)$ as a subset of $R^2$ defined as $x(t)=t,\quad y(t)=0.$

In [3]:

R2 = Manifold(2, 'R^2')             # manifold R^2
X.<x,y> = R2.chart()                # chart on R^2
                                    # F1: J -> R^2 :
F1 = J.continuous_map(R2,{(c, X):[t,0]}, name='F1')
                                    # plot c
p1=c.plot(X,mapping=F1,thickness=4,color='grey')
p1.show(figsize=[4,3])              # show plot

And next the image of $U$ under sinus function as a subset of $R^2$.

In [4]:

                                    # continuous map x=t,y=sin(t):
F2 = J.continuous_map(R2, {(c, X): [t,sin(t)]}, name='F2')
p=c.plot(X,mapping=F2,color='grey',thickness=2) # plot image of J
p.show(figsize=[4,3])                           # under F2

Example 3.2

Below, we map the same open interval into $R^3$.

In [5]:

# continuation
po1={'thickness':5,'color':'darkblue'}         # param.
po2={'fontsize':20,'color':'black'}

ax =line3d([(0,0,0), (1+0.15,0,0)], **po1)     # axes
ax+=line3d([(0,0,0), (0,1+0.15,0)], **po1)
ax+=line3d([(0,0,0), (0,0,6.28+0.15)], **po1)
ax+=text3d("x",(1.25,0,0),**po2)
ax+=text3d("y",(0,1.25,0),**po2)
ax+=text3d("z",(0.,0.,6.9),**po2)

In [6]:

J=manifolds.OpenInterval(0,2*pi)  # open interval as manifold
c.<t>=J.chart()                   # chart on J
R3 = Manifold(3, 'R^3')           # manifold R^3
X.<x,y,z> = R3.chart()            # chart on R^3
                                  # F: J -> R^3:
F=J.continuous_map(R3,{(c,X):[cos(3*t),sin(3*t),t]},name='F')
p=c.plot(X,mapping=F,color='grey',thickness=3,
  plot_points=200,label_axes=False)  # plot image of J
(p+ax).show(aspect_ratio=[1,1,0.3],frame=False)  # show plot

Two-dimensional function graph as a manifold¶

Example 3.3

Now let $U$ be an open rectangle $(-\pi,\pi)\times (-\pi,\pi)$ in $R^2$.

In [7]:

R2 = Manifold(2, 'R^2')                  # manifold R^2
c_xy.<x,y> = R2.chart()                  # chart on R^2 
U = R2.open_subset('U',                  # open subset of R^2
    coord_def={c_xy: [x>-pi,x<pi,y>-pi,y<pi]})
c_U.<x,y>=U.chart()                      # chart on U

First we plot the coordinate lines in the set $(-\pi,\pi)\times(-\pi,\pi)$.

In [8]:

p=c_U.plot(number_values={x:15,y:15},color='grey')  # plot
p.show(figsize=4)                        # coordinate lines on U

The graph of the function $f(x,y)=\cos(\sqrt{x^2+y^2})$ as a subset of $R^3$:

In [9]:

R3 = Manifold(3, 'R^3')                     # manifold R^3
c_XYZ.<X,Y,Z> = R3.chart()                  # chart on R^3
F = U.continuous_map(R3,                    # F: U -> R^3
  {(c_U, c_XYZ): [x,y,cos(sqrt(x^2+y^2))]}, name='F')
                    # plot the image of coordinate lines in  U:
p1=c_U.plot(c_XYZ,mapping=F,number_values={x:15,y:15},
  color='black',thickness=2,label_axes=False)
s=plot3d(cos(sqrt(x^2+y^2)),(x,-pi,pi),(y,-pi,pi),
         color='lightgrey',opacity=0.7)     # plot  image of U                 
(p1+s).rotateZ(-pi/6).show(frame=False,aspect_ratio=[1,1,1.8]) 
                                            # show plot