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# Riemannian geometry a beginners guide - Morgan F.

Morgan F. Riemannian geometry a beginners guide - Jones and Bartlett, 1993. - 121 p.
ISBN 0-86720-242-4
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Some famous minimal surfaces are pictured in Figures 3.2 through 3.4. At each point, since the mean curvature vanishes, the principal curvatures must be equal in magnitude and opposite in sign.
SURFACES IN R3 15
The catenoid
The helicoid y tan z = x Meusnier, 1776
yjx2 + y2 = cosh z Euler, 1740
Scherk’s surface cos y ez = cos x 1835
16 CHAPTER 3
x = Re(w — ^w3) y = Re(i(w + jw3)) z = Re(w2) we C
Figure 3.3. Enneper’s surface 1864.
3.4. Coordinates, length, metric. Local coordinates or parameters Mi, m2 on a C2 surface 5CR3 are provided by a C2 diffeomorphism (or parameterization) between a domain in the uu M2-plane and a portion of S.
For example, the standard spherical coordinates <p, 0 provide local coordinates on all of the sphere of radius a except for the poles (where the longitude 0 is undefined and q> is not differentiable). The position vector determined by these coordinates is
x = (x, y, z) = (a sin cp cos 0, a sin <p sin 0, a cos <p).
In general, the position is some function of the coordinates m, . Along a curve, these coordinates are in turn functions of a single parameter t.
Subscripts on the position vector x will denote partial derivatives
SURFACES IN R
Figure 3.4. The newly discovered complete, embedded minimal surface of Costa, Hoffman, and Meeks [Cos], [HoM], [Ho]. (Courtesy of David Hoffmann, Jim Hoffmann, and Michael Callahan.)
with respect to the iif:
dx _ / dy dz_
Xf = .
dlli \dlli dlli oUi
A dot will denote differentiation with respect to t:
x = — = 2 Xiiii (the chain rule). dt
18 CHAPTER 3
The arc length of a curve in the surface with coordinates u(t) is given by
L = J |x| dt = J |xiMi + x2ü2\ dt
Xi • Xi) Ml + 2(xi • X2)MiM2 + (*2 • X2) W2 dt (1)
^j* ”N/2 gijùi ùj dt.
where
dx ax
gij = xrXj = -——. (2)
dUi dUj
In other words, L = / ds, where
ds2 = X gij dui duj. (3)
For example, on the sphere of radius a, L = / ds, where, as it turns out,
ds2 = a2d(p2 + a2 sin2 (p dO2 = (<a2<p + a2 sin2 <p6) dt2,
so g 11 = a2, g22 — a2 sin2 <p, and gx2 = g2\ — 0 (see Exercise 3.2).
The matrix g = [gi;] is called the first fundamental form or metric. It is an intrinsic quantity in that it relates to measurements inside the surface. Notice that in the formula for length,
2 gijUiUj = guMiMi + g\2U\U2 + g21u2ux + g22u2u2
= gnul + 2g12M1M2 + g22«2.
For many surfaces in R3, it is convenient to use x, y as local coordinates and consider z(*,y). Then
Xi = (1,0, zx) and x2 = (0,1, zy).
The following proposition gives useful formulas for the mean curvature H and Gauss curvature G.
3.5. Proposition. For any local coordinates ux, u2 about a point p
SURFACES IN R3 19
in a C2 surface in R3, the second fundamental form II at p is similar to
g~l(Dh) -n^g~l
xn • n X12 • n L X12 • n X22 * n.
where
d x , Xi X x2
x„ = and n =
lJ dUidUj |xi X X2|
Consequently,
H = trace g~1(D2x) • n (1)
= x|xn - 2{xx « X2)Xi2 + XiX22 ^
Xixi - (xx • x2)2
G = det (g-1(D2x) • n) (2)
(Xn ♦ n)(x22 • n) - (Xi2 • n)2 X2x! - (Xx • x2)2
Before turning to the proof of Proposition 3.5, we note that
(A) Given H and G, you can solve easily for the principal curvatures:
H± V//2 - AG
k =----------------.
2
(B) If the surface is a graph x = (*, y, f(x, y)), then
rr = C1 +fy)fxx - 2fxfyfxy + (1 +fl)fyy
(1 +fl+fi)3n
^ fxxfyy-f
xy (A \
(4)
Proof. We may assume that S is tangent to the x, y-plane at p = 0, so S is locally a graph z = f(x, y) with /x(0) = fy(0) = 0 and n(0) = (0,0,1). For the particular local coordinates x, y,
x = (x,y,f(x,y)), g(0) = I.
20 CHAPTER 3
The proposition says that II is similar to
[fxx fxy L fxy /'vv J()
which is correct; indeed, they are equal.
Now let Mi, m2 be any local coordinates, and let J denote the Jacobian at 0:
/ =
Since
dx dX ~
dUi du2
dy_ dy_
-dU\ du2_
dz dZ
dui
0 dU2
= 0,
§ = JTJ.
Then by the chain rule, since — • n = 0 and — • n = 0,
dx dy
r i2.
g~1(D2x)-n = (JTJ)~1JT
is indeed similar to II.
rx
dx2
d2x
d2x dx dy d2x
_ dX dy dy"
Example. We will compute the curvature of the catenoid Vjc2 + y2 = cosh z of Figure 3.2. At most points we could use x and y as coordinates. Instead, we will use z and the polar coordinate 6. The equation says that r = cosh z. Hence the position
x = (jc, y, z) - (cosh z cos 6, cosh z sin 6, z)
Xi = (sinh z cos 6, sinh z sin 6,1) x2 = (-cosh z sin 6, cosh z cos 6, 0) xlx = (cosh z cos 6, cosh z sin 6, 0) x12 = (-sinh z sin 6, sinh z cos 6, 0) x22 = (-cosh z cos 6, -cosh z sin 6, 0)
SURFACES IN R3 21
(cos 0, sin 0, sinh z)
n = --------------------.
cosh z
By Proposition 3.5,
TT cosh2 z Xu - 0 + cosh2 z x22
H = • n = 0,
something
so the catenoid is indeed a minimal surface and Ki = — k2.
G = —----------------------= -cosh 4 z.
cosh z cosh z ~ 0
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