<p>"In general, a real-valued function of n variables is a rule f that assigns a real number f(x1, x2,..., xn) to each ordered n-tuple (x1, x2,..., xn) in a subset D of n-dimensional space."</p>
<ul>
<li> Calculus:One and several variables, Salas, Hille, and Etgen pg 821</li>
</ul>
<p>Your example is for radius = 1. The real equation is:</p>
<p>r(x,y,z)^2 = x^2 + y^2 +z^2</p>
<p>When you fix a radius, the problem becomes 2D.</p>
<p>"The term "sphere" refers to the surface only, so the usual sphere is a two-dimensional surface."</p>
<p>"That would be 4D since f is a function of the other variables."</p>
<p>I don't believe this. Although a 5D only holds 3 spacial dimensions, one time dimension, and a fifth parameter that varies with all other 4 dimensions.</p>
<p>It's like saying z = x + y is 2D because z depends on x and y. </p>
<p>Flow fields are 4D. Time parameterized flow fields are 5D.</p>
<p>the two-dimensional surface refers to when you unfold the sphere into a planar surface because there is nothing in between the center and the surface. however, you cannot define a sphere without the use of 3 coordinates. you can just as easily define a "ball" which is a 3D object if you supplement the = sign with an inequality. because a sphere is graphed in R3, it is considered 3 dimensional.</p>
<p>Out of Calc I, Calc II, Calc III & Diff Eq....I would say Calc III.</p>
<p>Then again (and I was a math major as an undergrad)...I hate vectors.</p>
<p>The hardest Calculus class period? That would be Advanced Calculus which was almost all proofs. I still remember the prof of that course trying have us prove the A times Zero = Zero. That is a 10-step proof using axioms. I was like "I am paying you to teach me this???"</p>
<p>
[quote]
the two-dimensional surface refers to when you unfold the sphere into a planar surface because there is nothing in between the center and the surface. however, you cannot define a sphere without the use of 3 coordinates. you can just as easily define a "ball" which is a 3D object if you supplement the = sign with an inequality. because a sphere is graphed in R3, it is considered 3 dimensional.
[/quote]
</p>
<p>Not that it really matters, or is even on topic, but no. Spheres are 2-dimensional. The reasoning becomes crystal-clear when looking at it from a linear algebra + differential geometry point of view(which I will not descend into). Similarly, (x, y, z, f(x, y, z), t) is 4-D.</p>
<p>Now, on-topic. PDE's (partial differential equations) is probably the hardest math for me, partly because I have the least experience with the, partially because you have to do lots of practice problems to master them, and finally because they're just plain boring from a conceptual point of view.</p>
<p>Lots of people don't like Calc II and III, but I rather enjoyed calc 3.</p>
<p>I hate the one I'm in now (Calc I). They teach the class as if everyone has had calculus before, but I haven't had it. Also, no calculators (though, you don't really NEED them, but they would make a few things easier).</p>
<p>"Not that it really matters, or is even on topic, but no. Spheres are 2-dimensional. The reasoning becomes crystal-clear when looking at it from a linear algebra + differential geometry point of view(which I will not descend into). Similarly, (x, y, z, f(x, y, z), t) is 4-D."</p>