## 2018年3月22日木曜日

### 数学 - Python - JavaScript - 解析学 - 積分 - 積分の性質 - 広義の積分(対数関数、判定、収束しない場合)

1. $\begin{array}{}\frac{1}{{x}^{2}-2x}\\ =\frac{1}{x\left(x-2\right)}\\ \frac{A}{x}+\frac{B}{x-2}\\ =\frac{\left(A+B\right)x-2A}{x\left(x-2\right)}\\ A+B=0\\ -2A=1\\ A=-\frac{1}{2}\\ B=\frac{1}{2}\end{array}$
$\begin{array}{}{\int }_{0}^{2}\frac{1}{{x}^{2}-2x}\mathrm{dx}\\ =\frac{1}{2}{\int }_{0}^{2}\left(-\frac{1}{x}+\frac{1}{x-2}\right)\mathrm{dx}\\ =\frac{1}{2}\left(-{\int }_{0}^{2}\frac{1}{x}\mathrm{dx}+{\int }_{0}^{2}\frac{1}{x-2}\mathrm{dx}\right)\\ =\frac{1}{2}\left(-\underset{t\to 0}{\mathrm{lim}}{\int }_{t}^{2}\frac{1}{x}\mathrm{dx}+\underset{t\to 0}{\mathrm{lim}}\underset{0}{\overset{2-t}{\int }}\frac{1}{x-2}\mathrm{dx}\right)\\ =\frac{1}{2}\left(-\underset{t\to 0}{\mathrm{lim}}{\left[\mathrm{log}x\right]}_{t}^{2}+\underset{t\to 0}{\mathrm{lim}}{\left[\mathrm{log}\left(2-x\right)\right]}_{0}^{\left(2-t\right)}\right)\\ =\frac{1}{2}\left(-\underset{t\to 0}{\mathrm{lim}}\left(\mathrm{log}2-\mathrm{log}t\right)+\underset{t\to 0}{\mathrm{lim}}\left(\mathrm{log}t-\mathrm{log}2\right)\right)\\ =\frac{1}{2}\left(-\mathrm{log}2-\infty -\infty -\mathrm{log}2\right)\\ =-\infty \end{array}$

よって収束しない。

コード(Emacs)

Python 3

#!/usr/bin/env python3
from sympy import pprint, symbols, oo, Integral, plot

x = symbols('x')
f = 1 / (x ** 2 - 2 * x)
I = Integral(f, (x, 0, 2))

for t in [f, I, I.doit()]:
pprint(t)
print()

p = plot(f, legend=True, show=False)

p.save('sample5.svg')


$./sample5.py 1 ──────── 2 x - 2⋅x 2 ⌠ ⎮ 1 ⎮ ──────── dx ⎮ 2 ⎮ x - 2⋅x ⌡ 0 -∞$


HTML5

<div id="graph0"></div>
<pre id="output0"></pre>
<label for="r0">r = </label>
<input id="r0" type="number" min="0" value="0.5">
<label for="dx">dx = </label>
<input id="dx" type="number" min="0" step="0.001" value="0.001">
<br>
<label for="x1">x1 = </label>
<input id="x1" type="number" value="-5">
<label for="x2">x2 = </label>
<input id="x2" type="number" value="5">
<br>
<label for="y1">y1 = </label>
<input id="y1" type="number" value="-5">
<label for="y2">y2 = </label>
<input id="y2" type="number" value="5">

<button id="draw0">draw</button>
<button id="clear0">clear</button>

<script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/d3/4.2.6/d3.min.js" integrity="sha256-5idA201uSwHAROtCops7codXJ0vja+6wbBrZdQ6ETQc=" crossorigin="anonymous"></script>

<script src="sample5.js"></script>


JavaScript

let div0 = document.querySelector('#graph0'),
pre0 = document.querySelector('#output0'),
width = 600,
height = 600,
btn0 = document.querySelector('#draw0'),
btn1 = document.querySelector('#clear0'),
input_r = document.querySelector('#r0'),
input_dx = document.querySelector('#dx'),
input_x1 = document.querySelector('#x1'),
input_x2 = document.querySelector('#x2'),
input_y1 = document.querySelector('#y1'),
input_y2 = document.querySelector('#y2'),
inputs = [input_r, input_dx, input_x1, input_x2, input_y1, input_y2],
p = (x) => pre0.textContent += x + '\n',
range = (start, end, step=1) => {
let res = [];
for (let i = start; i < end; i += step) {
res.push(i);
}
return res;
};

let f = (x) => 1 / (x ** 2 - 2 * x),
fns = [[f, 'red']];

let draw = () => {
pre0.textContent = '';

let r = parseFloat(input_r.value),
dx = parseFloat(input_dx.value),
x1 = parseFloat(input_x1.value),
x2 = parseFloat(input_x2.value),
y1 = parseFloat(input_y1.value),
y2 = parseFloat(input_y2.value);

if (r === 0 || dx === 0 || x1 > x2 || y1 > y2) {
return;
}

let points = [],
lines = [[0, y1, 0, y2, 'green'],
[2, y1, 2, y2, 'blue']],
fns1 = [],
fns2 = [];

fns
.forEach((o) => {
let [f, color] = o;
for (let x = x1; x <= x2; x += dx) {
let y = f(x);

points.push([x, y, color]);
}
});

fns1
.forEach((o) => {
let [f, color] = o;

lines.push([x1, f(x1), x2, f(x2), color]);
});

fns2
.forEach((o) => {
let [f, color] = o;
for (let x = x1; x <= x2; x += dx0) {
let g = f(x);
lines.push([x1, g(x1), x2, g(x2), color]);
}
});

let xscale = d3.scaleLinear()
.domain([x1, x2])
let yscale = d3.scaleLinear()
.domain([y1, y2])

let xaxis = d3.axisBottom().scale(xscale);
let yaxis = d3.axisLeft().scale(yscale);
div0.innerHTML = '';
let svg = d3.select('#graph0')
.append('svg')
.attr('width', width)
.attr('height', height);

svg.selectAll('line')
.data([[x1, 0, x2, 0], [0, y1, 0, y2]].concat(lines))
.enter()
.append('line')
.attr('x1', (d) => xscale(d[0]))
.attr('y1', (d) => yscale(d[1]))
.attr('x2', (d) => xscale(d[2]))
.attr('y2', (d) => yscale(d[3]))
.attr('stroke', (d) => d[4] || 'black');

svg.selectAll('circle')
.data(points)
.enter()
.append('circle')
.attr('cx', (d) => xscale(d[0]))
.attr('cy', (d) => yscale(d[1]))
.attr('r', r)
.attr('fill', (d) => d[2] || 'green');

svg.append('g')
.attr('transform', translate(0, ${height - padding})) .call(xaxis); svg.append('g') .attr('transform', translate(${padding}, 0))
.call(yaxis);

[fns, fns1, fns2].forEach((fs) => p(fs.join('\n')));
};

inputs.forEach((input) => input.onchange = draw);
btn0.onclick = draw;
btn1.onclick = () => pre0.textContent = '';
draw();