7. Smooth functions and pullbacks

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

In [1]:
'SageMath version 9.6, Release Date: 2022-05-15'

Recall that if $M$ is a smooth manifold, the set of all smooth functions from $M$ to $R$ is denoted by $C^∞(M)$. This set is a ring with the operations given by
\begin{equation} \begin{matrix} ( f + g)( p) = f ( p) + g( p),\\ ( f g)( p) = f ( p)g( p), \end{matrix} \label{}\tag{7.1} \end{equation}
for $f, g ∈ C^\infty(M),\ \ a ∈ R$ and $p ∈ M.$

One can also define an additional operation $$(a f )( p) = a f ( p),$$

for $f∈ C^\infty(M),\ \ a ∈ R$ and $p ∈ M.$

If $ψ$ is a smooth map from $M$ to a smooth manifold $N$ and $f ∈ C^∞ (N )$, the pullback $ \ \ ψ ^* f\ \ $ of $f$ under $\ \ ψ\ \ $ is defined by

\begin{equation} ψ^∗ f = f ◦ ψ. \label{}\tag{7.2} \end{equation}

If $\phi\ $ and $\ \chi\ $ are charts on $M$ and $N$, respectively, then from $\ \ (ψ^∗ f ) ◦ φ^{−1} = ( f ◦ χ^{−1} ) ◦ (χ ◦ ψ ◦ φ^{−1} )\ \ $ it follows that $ψ^∗ f ∈ C^∞ (M).$

The operation $ψ^∗$ is applied to functions defined on $N$ to produce functions defined on $M$, hence the name pullback for $ψ^∗$.

Example 7.1

If we denote by $\psi$ the coordinate change defined by $x=r\cos(\phi),\; y=r\sin(\phi)$ on the open set $U=\{(r,\phi): r>0, 0<\phi <2\pi \},$ then for a scalar function $f$ of variables $x,y,$ the pullback $\psi^*f$ is the function $(r,\phi)\to f(r\cos(\phi),r\sin(\phi))$, defined on $U$.

In [2]:
%display latex
M = Manifold(2, 'R^2p')   # manifold R^2
U = M.open_subset('U')    # open subset of R2, 
                          # polar coord. on U: 
c_rphi.<r,phi>=U.chart(r'r:(0,oo) phi:(0,2*pi):\phi')
N = Manifold(2, 'R^2c')   # second copy of R^2                         
c_xy.<x,y> = N.chart()    # Cartesian coordinates:
                          # psi: M --> N : polar to Cart.:
psi = U.diff_map(N, (r*cos(phi), r*sin(phi)),name=r'\psi')
                          # f -scalar function on N 
f = N.scalar_field(function('f')(x,y), name='f') 

fplb=psi.pullback(f)      # psi^*f -scalar func. on M
fplb.disp()               # show pullback
\(\displaystyle \begin{array}{llcl} {\psi}^*f:& U & \longrightarrow & \mathbb{R} \\ & \left(r, {\phi}\right) & \longmapsto & f\left(r \cos\left({\phi}\right), r \sin\left({\phi}\right)\right) \end{array}\)

If $ψ$ is a smooth map $M\to N$ then \begin{equation} ψ^∗ (a f + bg) = aψ^∗ f + bψ^∗ g, \tag{7.3} \end{equation}
for $f, g ∈ C^∞ (N )$ and $a, b ∈ R.$

In fact $$ψ^∗ (a f + bg) ( p) = (a f + bg)(ψ( p)) = a f (ψ( p)) + bg(ψ( p))\\ = a(ψ^∗ f )( p) + b(ψ^∗ g)( p) = (aψ^∗ f + bψ^∗ g)( p). $$

If $ψ$ is a smooth map $M\to N$ then \begin{equation} ψ^∗ ( f g) = (ψ^∗ f )(ψ^∗ g), \tag{7.4} \end{equation}
for $f, g ∈ C^∞ (N )$.

The last relation follows from

$$ψ^∗ ( f g) ( p) = ( f g)(ψ( p)) = f (ψ( p))g(ψ( p)) = (ψ^∗ f )( p)(ψ^∗ g)( p) = (ψ^∗ f )(ψ^∗ g) ( p). $$

If $M_1,M_2,M_3$ are smooth manifolds and $ψ_1 : M_1 → M_2$ and $ψ_2 : M_2 → M_3$ are smooth maps, then

\begin{equation} (ψ_2 ◦ ψ_1 )^∗ = ψ_1^∗ ◦ ψ_2^∗ . \tag{7.5} \end{equation}

This is consequence of

$$(ψ_2 ◦ ψ_1 )^∗ f = f ◦ (ψ_2 ◦ ψ_1 ) = ( f ◦ ψ_2 ) ◦ ψ_1 = (ψ_2^∗ f ) ◦ ψ_1 = ψ_1^∗ (ψ_2^∗ f )= (ψ_1^∗ ◦ ψ_2^∗ ) f. $$

Curves in a manifold

Let $M$ be a smooth manifold. A smooth curve $\gamma$ in $M$, is a smooth mapping $\gamma : I → M$ where $I\subset R$ is an open interval.
Equivalently: $\gamma : I → M$ is a smooth curve in $M$ if $I$ is an open interval of $R$ and $φ ◦ \gamma: I\to R^n$ is a smooth map for every chart $(U, φ)$ of the atlas of $M$.

In local coordinates $\ \ (x^1,\ldots,x^n)\ $ the curve is defined if we define $\ n\ $ real functions

$$\gamma^1(t)=x^1(\gamma(t)),\ldots, \gamma^n(t)=x^n(\gamma(t)),\quad t\in I. $$

Example 7.2

Assume, that in an open subset $U\subset R^2$ we have a curve defined in polar coordinates: $\ r=1,\phi=t, \ t\in(0,2\pi)$.
Since in this example the transition maps are defined, the representation of the curve in Cartesian coordinates is determined by the system.

In [3]:
M = Manifold(2, 'R^2', r'\RR^2')  # the Euclidean plane R^2
c_xy.<x,y> = M.chart()            # Cartesian coordinates on R^2
U = M.open_subset('U', coord_def={c_xy: (y!=0, x<0)}) 
                   # the complement of the segment y=0 and x>0
c_cart = c_xy.restrict(U)         # Cartesian coordinates on U
                                  # spherical coordinates on U:
c_spher.<r,ph> = U.chart(r'r:(0,+oo) ph:(0,2*pi):\phi') 
                                  # transition spher. to Cartesian
ch_cart_spher=c_cart.transition_map(c_spher, [sqrt(x*x+y*y),
                                  # transition Cartesian to spher.
ch_cart_spher.set_inverse(r*cos(ph), r*sin(ph))
R.<t> = manifolds.RealLine()      # manifold R
c = U.curve({c_spher: (1,t)}, (t, 0, 2*pi), name='c') # curve c
print("Representations in polar and Cartesian coordinates:")
show(c.coord_expr(c_spher))       # curve c in spherical coord.
show(c.coord_expr(c_cart))        # curve c in Cartesian coord.
cp=c.plot(color='black',aspect_ratio=1)   # curve plot
cp.show(figsize=[3,3])            # show plot
Check of the inverse coordinate transformation:
  x == x  *passed*
  y == y  *passed*
  r == r  *passed*
  ph == arctan2(r*sin(ph), r*cos(ph))  **failed**
NB: a failed report can reflect a mere lack of simplification.

Representations in polar and Cartesian coordinates:
\(\displaystyle \left(1, t\right)\)
\(\displaystyle \left(\cos\left(t\right), \sin\left(t\right)\right)\)

Example 7.3

Consider the curve in $R^3$ defined in Cartesian coordinates by $\ x=\cos 3t,\ y=\sin 3t,\ z=2t$:

In [4]:
J = manifolds.OpenInterval(0, 2*pi)      # open interval
c.<t>=J.chart()                          # coordinate t on c
R3 = Manifold(3, 'R^3')                  # manifold R^3
X.<x,y,z> = R3.chart()                   # Cartesian coord. on R^3
                                         # curve c in R^3:
c = R3.curve({X:[cos(3*t),sin(3*t),2*t]},(t,-pi/2,+pi/2),name='c')
                                         # plot c:
p = c.plot(X,color='grey',thickness=4,plot_points=200,label_axes=False)
p.show(aspect_ratio=[1,1,0.3])           # show plot

Example 7.4

Define a curve on the sphere $S^2$, using spherical coordinates $\ \theta=t,\ \phi=\cos 2t$:

In [5]:
%display latex
S2=manifolds.Sphere(2)                 # sphere S^2
Phi=S2.embedding()                     # embedding S^2-> E^3
sph.<th,ph>=S2.spherical_coordinates() # spherical coord. on S^2
E=S2.ambient()                         # Euclidean space E^3
cart.<x,y,z> = E.cartesian_coordinates() # Cartesian coord. on E^3
fun=Phi.expr()                         # functions defining Phi
R.<t> = manifolds.RealLine()           # Real line as manifold
c = S2.curve({sph: [t, cos(2*t)]}, (t, -oo, +oo),
             name='c')                 # curve c
                                       # plot c:
          plot_points=200, aspect_ratio=1,label_axes=False)
        color='lightgrey',opacity=0.7) # plot sphere
(p1+p2).rotateZ(pi/2).show(frame=False,label_axes=False) # show plot

Tangent vector to a smooth curve

If $\gamma$ is a smooth curve in $M$ and $f ∈ C^∞ (M),$ then $\gamma^∗ f = f ◦ \gamma$ is a smooth function from an open interval $I ⊂ R$ into $R$. If $t_0 ∈ I$ , the tangent vector to $\gamma$ at the point $\gamma(t_0 )$, denoted by $\gamma'_{t_0}$ , is the map $C^\infty(M)\to R$ defined by \begin{equation} \gamma'_{t_0}(f) =\frac{d}{dt}(\gamma^*f)\Big|_{t_0}= \frac{d}{dt}(f\circ\gamma)\Big|_{t_0},\quad f\in C^\infty(M). \label{}\tag{7.6} \end{equation}

Using (7.3), (7.4) we can check that $\gamma'_{t_0}:C^∞ (M)\to R$ has the properties $$\gamma'_{t_0}(af+bg)=\frac{d}{dt}(\gamma^*(af+bg))\Big|_{t_0}=\frac{d}{dt}(a\gamma^*(f)+b\gamma^*(g))\Big|_{t_0}\\ =a\frac{d}{dt}(\gamma^*f)\Big|_{t_0}+ b\frac{d}{dt}(\gamma^*g)\Big|_{t_0}= a\gamma'_{t_0}(f)+b\gamma'_{t_0}(f), $$ and $$\gamma'_{t_0}(fg)=\frac{d}{dt}(\gamma^*(fg))\Big|_{t_0}=\frac{d}{dt}(\gamma^*(f)\gamma^*(g))\Big|_{t_0}\\ =f (\gamma(t_0 ))\frac{d}{dt}(\gamma^*(g))\Big|_{t_0} +g (\gamma(t_0 ))\frac{d}{dt}(\gamma^*(f))\Big|_{t_0}\\ =f (\gamma(t_0 ))\gamma'_{t_0}(g)+ g (\gamma(t_0 ))\gamma'_{t_0}(f), $$
for $f, g ∈ C^∞ (M)$ and $a,b\in R$.

What's next?

Take a look at the notebook Tangent spaces.