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# Tag: division

## graphs – What is the difference between Partition and Division?

**I will explain this with an example :**

*Consider a set $S$ which contains a collections of sets with a constant k such that:*

$$ S = { {S_1}, {S_2}, {S_3},{S_4},……….{S_k} }$$

### In Partition of Set S :

Then we can form the set again by

$ S=S_1cup S_2cup S_3cup S_4………..cup S_k$

$where S_icap S_j = phi$

$and ineq j and i,jin mathcal{N} $

### In Division of Set S :

We just have this

$ S=S_1cup S_2cup S_3cup S_4………..cup S_k$

## Double lines division representation

How to make a fraction to display a numerator on top of the denominator in Google Sheet?

## ac.commutative algebra – Rings with terminating division chains of a given length

Let $R$ be an integral domain. Given $a,bin R$, then a division chain for $(a,b)$ is a sequence where we take $r_{-1}=a$, $r_0=b$, and for each $n>0$ we take $r_n=r_{n-1}s_n+r_{n-2}$ for some $s_nin R$. We say that the division chain terminates if $r_n=0$, and it terminates at length $n$ when $r_ineq 0$ for $i<n$.

These concepts, of course, have a lot to do with Euclidean domains, etc…

I ran across the following very interesting fact, proved by Cooke and Weinberger in 1975. Let $K$ be a number field, and let $R=mathscr{O}_{K,S}$, the ring of $S$-integers, where $S$ contains all the infinite places of $K$. Assume (some appropriate version of) GRH. If the unit group of $R$ is infinite, and $aR+bR=R$, then there is a terminating division chain for the pair $(a,b)$ of length $5$. Under some additional restrictions, like $S$ has a non-infinite place, or $K$ has a real embedding, the number $5$ can be lowered to $4$ or $3$. Moreover, they give examples showing that these numbers are best possible in some cases.

I’m interested if the following appears anywhere in the literature: For each $n>5$, there exists an integral domain $R$ such that for any $a,bin R$ with $aR+bR=R$, there is a terminating division chain for the pair $(a,b)$ of length $n$; and there is some pair of comaximal elements in $R$ that doesn’t have a smaller terminating length. The example will have to be somewhat complicated, since it won’t be a ring of integers over a number field.

## python – Integer division when implementing Karatsuba’s algorithm

So im currently implementing Karatsuba’s algorithm in python and I cam across a very odd issue. I will provide two sets of code and I believe that they both theoretically do the same thing, but provide different answers (one right and one wrong).

The first set of code:

```
def decompose(x, n):
a = int(((x)//10**(n)))
b = int(x % (10**(n)))
#print("a", a, "t", "b", b)
return a, b
def digits(x):
return len(str(x))
# both x and y are n digit numbers
def multiply(x, y):
n2 = min(digits(x), digits(y))//2
if x < 10 or y < 10:
return x*y
a, b = decompose(x, n2)
# print("a", a, "t", "b", b)
c, d = decompose(y, n2)
# print("c", c, "t", "d", d)
if min(digits(a), digits(b), digits(c), digits(d)) == 1:
ac = int(a*c)
bd = int(b*d)
adbc = int((a+b)*(c+d) - a*c - b*d)
else:
ac = multiply(a, c)
bd = multiply(b, d)
adbc = multiply(a+b, c+d) - ac - bd
return (10**(2*n2))*ac + (10**(n2))*(adbc) + bd
x, y = 3141592653589793238462643383279502884197169399375105820974944592, 2718281828459045235360287471352662497757247093699959574966967627
print(multiply(x, y))
print(x*y)
```

Second set of code:

```
def decompose(x):
n = digits(x)
a = int(((x)//10**(n//2)))
b = int(x % (10**(n//2)))
#print("a", a, "t", "b", b)
return a, b
def digits(x):
return int(len(str(x)))
# both x and y are n digit numbers
def multiply(x, y):
n2 = min(digits(x), digits(y))//2
if x < 10 or y < 10:
return x*y
a, b = decompose(x)
# print("a", a, "t", "b", b)
c, d = decompose(y)
# print("c", c, "t", "d", d)
if min(digits(a), digits(b), digits(c), digits(d)) == 1:
ac = int(a*c)
bd = int(b*d)
adbc = int((a+b)*(c+d) - a*c - b*d)
else:
ac = multiply(a, c)
bd = multiply(b, d)
adbc = multiply(a+b, c+d) - ac - bd
return (10**(2*n2))*ac + (10**(n2))*(adbc) + bd
x, y = 3141592653589793238462643383279502884197169399375105820974944592, 2718281828459045235360287471352662497757247093699959574966967627
print(multiply(x, y))
print(x*y)
```

In the first set, I pass n2, where n2 = 32 because x and y have 64 digits. In this case I get the correct answer for the product of x and y. However, in the second set of code, I manually find the number of digits and then divide it by 2 using integer division, which should also give 32 but gives me the wrong answer. Any help would be appreciated.

## nt.number theory – Explicit construction of division algebras of degree 3 over Q

In his book *Introduction to arithmetic groups*, Dave Witte Morris implicitly gives a construction of central division algebras of degree 3 over $mathbb{Q}$ in Proposition 6.7.4. More precisely, let $L/mathbb{Q}$ be a cubic Galois extension and $sigma$ a generator of its Galois group.If $p in mathbb{Z}^+$ and $p neq tsigma(t)sigma^2(t)$ for all $t in L$, then

$$ D=left{ begin{pmatrix}

x & y & z\

psigma(z) & sigma(x) & sigma(y)\

psigma^2(y) & psigma^2(z) & sigma^2(x)

end{pmatrix} :(x,y,z)in L^3 right}

$$

is a division algebra.

On page 145, just before Proposition 6.8.8, Morris claims that it is knows that every division algebra of degree 3 arises in this manner. This should follow from the fact that every central division algebra of degree 3 is cyclic. I could not find this explicit construction in my references (e.g. Pierce – *Associative Algebras*, though maybe I missed something) and I would like to know if there is a reference or a quick way to see that this exhausts all central division algebras of degree 3 over $mathbb{Q}$.

## nt.number theory – Multiplication law in a division algebra of dimension 9 over a non-archimedean local field

Let $k$ be a non-archimedean local field, for example, a $p$-adic field (a finite extension of the filed ${Bbb Q}_p$ of $p$-adic numbers).

It is well known that there is a canonical isomorphism

$${rm inv}colon {rm Br},koversetsimlongrightarrow{Bbb Q}/{Bbb Z}.$$

Let $D$ denote the division algebra of dimension 9 over $k$ with invariant $frac13$.

Question.How can one explicitly describe the multiplication law in $D$ ?

**Motivation.** From the multiplication law in $D$, I can obtain the commutation law in the 8-dimensional Lie algebra ${frak g}={frak sl}(1,D)$.

From $frak g$, I can obtain an explicit trilinear alternating form on the 8-dimensional space $frak g$:

$$(x,y,z)mapsto ((x,y),z)quadtext{for },x,y,zin{frak g},$$

where $(,,,)$ denotes the Killing form.

This is a $k$-form of a generic alternating trilinear form on $k^8$.

## División por zero en php

tengo una función que me calcula los likes y dislikes, para luego hacer un for con estrellas para el tema del rating. Pero tengo el problema de que si tiene 0 likes y 0 dislikes, luego al dividir salta un error de división por cero:

Esta es mi función:

```
function devolverRating($likes,$dislikes){
return round($likes/($likes+$dislikes)*5);
}
```

Y el for con el que pinto las estrellas en función del rating que me devuelve, pero hay veces que a lo mejor me devuelve un 2 y me pinta las 5 estrellas o que tiene de rating 0 y me pinta 5 estrellas, sin contar lo de división por cero:

```
for($i=0;$i<5;$i++){
if($i<=$rating){
echo "<i class='fas fa-star'></i>";
}
else{
echo"<i class='far fa-star'></i>";
}
}
```

## python – Complex division for imaginary part

I seek a fast implementation of `(X / Y).imag`

, where `X, Y`

are complex 2D arrays (PyTorch tensors already on GPU). My approach is to move computation to a CUDA kernel via `cupy`

, interfacing C and Python. Requirements:

- Faster than
`(X / Y).imag`

- Using same or less memory than
`(X / Y).imag`

- Output is same as
`(X / Y).imag`

within float precision `#include`

s must be supported by CuPy (which is largely limited to standard CUDA libraries afaik)

**My attempt**:

```
extern "C" __global__
void cdiv_imag(float x(240)(240000)(2), float y(240)(240000)(2),
float out(240)(240000)) {
int i = blockIdx.x * blockDim.x + threadIdx.x;
int j = blockIdx.y * blockDim.y + threadIdx.y;
if (i >= 240 || j >= 240000)
return;
float A = x(i)(j)(0);
float B = x(i)(j)(1);
float C = y(i)(j)(0);
float D = y(i)(j)(1);
out(i)(j) = (B*C - A*D) / (C*C + D*D);
}
```

This uses half the memory, since `X / Y`

yields a temp array of reals & imaginary, but is still ultimately slower per my benchmarks on GTX 1070:

```
0.02320581099999992 sec (avg of 1000 runs)
0.01924530900000002 sec (avg of 1000 runs)
```

Full code; Win 10 x64, Python 3.7.9, CuPy 8.3.0, PyTorch 1.8.0.

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