Wikipedia:Reference desk/Archives/Mathematics/2014 April 28

= April 28 =

Expression for a Gamma-like Function
Let $$\Gamma_a(x+a)=x\cdot\Gamma_a(x).$$ Then for $$a=1$$ we have $$\Gamma_{_1}(x)=\Gamma(x),$$ whose integral expression is $$\int_0^\infty\frac{t^{x-1}}{e^t}dt.$$ I was wondering whether such expressions also exist for this generalized version of the $$\Gamma$$ function. — 79.113.194.139 (talk) 04:30, 28 April 2014 (UTC)
 * Yes, it's just $$\Gamma_a(x)=\int_0^\infty\frac{t^{x-a}}{e^t}dt.$$ Widener (talk) 08:27, 28 April 2014 (UTC)
 * I think you answered the question that was literally asked, but perhaps not the one that was intended. My guess is that the OP meant to write
 * $$\Gamma_a(x+a)=x\cdot\Gamma_a(x)$$
 * How about it, 79.113? Did I guess right? --Trovatore (talk) 09:15, 28 April 2014 (UTC)
 * Yes. Sorry. — 79.113.194.139 (talk) 09:44, 28 April 2014 (UTC)
 * Then that recursive relationship doesn't uniquely define $$\Gamma_a$$. What's the boundary condition? The gamma function was motivated to coincide with the factorial. What is the motivation behind $$\Gamma_a$$? 203.45.159.248 (talk) 09:59, 28 April 2014 (UTC)
 * Assuming you also want $$\Gamma_a(0)=1$$, then $$\Gamma_a(x) = a^{x/a} \Gamma(x/a)$$.  Sławomir Biały  (talk) 11:31, 28 April 2014 (UTC)
 * Wow! Unbelievable as always, Slawomir! :-) If you could you also please explain the logic and/or intuition which helped you arrive at this expression ? Thanks! — 86.125.209.133 (talk) 17:56, 28 April 2014 (UTC)
 * From the equation $$\Gamma_a(x+a)=x\Gamma_a(x)$$, I see that the solution should be something like $$\Gamma(x/a)$$. This doesn't quite work, so I try to write $$\Gamma_a(x) = f(x)\Gamma(x/a)$$.  Imposing the functional equation again gives $$f(x+a)=af(x)$$ which has $$f(x)=a^{x/a}$$ as a solution.  (Clearly there will be many functions solving this; I'm not sure what conditions are needed to ensure uniqueness.)   Sławomir Biały  (talk) 22:02, 28 April 2014 (UTC)

A Space with Space-filling Dodecahedrons ???!!!...
The Cube is a Space-filling Solid...

The Dodecahedron is NOT!!!

Four Dodecahedrons have a empty Corner: 0xyz with 1,5o

Could it be a SPACE with Space-filling Dodecahedrons???...

Could it be a SPACE where the Icosahedron = 20 Tetrahedrons???...

I can Imagine them BUT could they be Calculated???...

Are these "Mistakes" say something about our Universe???...

THANK you VERY-VERY much!!!...

"Have a nice Day/Night..."

SPYROY Kosta - Greece - Honeycomp (talk) 14:36, 28 April 2014 (UTC)


 * In hyperbolic space, you can tile appropriately scaled dodecahedra. Their dihedral angles vary according to their size, so you can make them have dihedral angles of precisely 90° (order-4 dodecahedral honeycomb), 72° (order-5 dodecahedral honeycomb), 60° (order-6 dodecahedral honeycomb), ..., 0° (infinite-order dodecahedral honeycomb). In elliptic space, you can also tile appropriately scaled dodecahedra 3 ({5,3,3}, 120-cell) or 2 ({5,3,2}, dodecahedral dichoron, each dodecahedron takes up a 3-hemisphere) at a corner. Double sharp (talk) 14:47, 28 April 2014 (UTC)
 * Though if the order is more than 6 the vertices stick out beyond infinity. And as to {5,3,2}, how do you distinguish it from a great sphere? —Tamfang (talk) 03:13, 29 April 2014 (UTC)
 * It's analogous to the pentagonal dihedron {5,2}: the faces of the dodecahedra in {5,3,2} tile a great sphere, just as the sides of the pentagons in {5,2} tile a great circle. Double sharp (talk) 15:24, 29 April 2014 (UTC)


 * I don't think this says anything significant about our universe - Euclidean 3-space is only a local approximation of its actual geometry. There's only one space-filling fully regular honeycomb in 3-space - the cube, which has three sets of parallel faces. So if you want to consider the philosophical impact of this, I guess the question is "Why cubes?" AlexTiefling (talk) 15:57, 28 April 2014 (UTC)


 * You are probably thinking of the rhombic dodecahedron. This is well-known to fill 3-space, and can be thought of as the 3-space analogy of the regular hexagon (which fills (tessellates) 2-space), though it is not regular. RomanSpa (talk) 19:11, 28 April 2014 (UTC)
 * It's sufficiently clear, in my humble opinion, that that's not what he has in mind. —Tamfang (talk) 03:13, 29 April 2014 (UTC)


 * Here are two views of a curved space filled with regular dodecahedra  — and two views of a space where 20 tetrahedra form an icosahedron   —Tamfang (talk) 06:08, 29 April 2014 (UTC)