改良オイラー法計算機

修正オイラー法を用いて、$$$y{\left(0 \right)} = 1$$$、$$$h = \frac{1}{5}$$$ が与えられているとき、$$$y^{\prime }\left(t\right) = 2 t - y$$$ に対して $$$y{\left(1 \right)}$$$ を求めよ。

修正オイラー法によれば $$$y_{n+1} = y_{n} + h f{\left(t_{n} + \frac{h}{2},y_{n} + \frac{h}{2} f{\left(t_{n},y_{n} \right)} \right)}$$$、ここで $$$t_{n+1} = t_{n} + h$$$。

$$$h = \frac{1}{5}$$$, $$$t_{0} = 0$$$、$$$y_{0} = 1$$$、および$$$f{\left(t,y \right)} = 2 t - y$$$が成り立つ。

ステップ 1

$$$t_{1} = t_{0} + h = 0 + \frac{1}{5} = \frac{1}{5}$$$

$$$f{\left(t_{0},y_{0} \right)} = f{\left(0,1 \right)} = -1$$$

$$$y_{1} = y{\left(t_{1} \right)} = y{\left(\frac{1}{5} \right)} = y_{0} + h f{\left(t_{0} + \frac{h}{2},y_{0} + \frac{h}{2} f{\left(t_{0},y_{0} \right)} \right)} = 1 + \frac{f{\left(0 + \frac{\frac{1}{5}}{2},1 + \frac{\frac{1}{5}}{2} \left(-1\right) \right)}}{5} = 0.86$$$

ステップ 2

$$$t_{2} = t_{1} + h = \frac{1}{5} + \frac{1}{5} = \frac{2}{5}$$$

$$$f{\left(t_{1},y_{1} \right)} = f{\left(\frac{1}{5},0.86 \right)} = -0.46$$$

$$$y_{2} = y{\left(t_{2} \right)} = y{\left(\frac{2}{5} \right)} = y_{1} + h f{\left(t_{1} + \frac{h}{2},y_{1} + \frac{h}{2} f{\left(t_{1},y_{1} \right)} \right)} = 0.86 + \frac{f{\left(\frac{1}{5} + \frac{\frac{1}{5}}{2},0.86 + \frac{\frac{1}{5}}{2} \left(-0.46\right) \right)}}{5} = 0.8172$$$

ステップ 3

$$$t_{3} = t_{2} + h = \frac{2}{5} + \frac{1}{5} = \frac{3}{5}$$$

$$$f{\left(t_{2},y_{2} \right)} = f{\left(\frac{2}{5},0.8172 \right)} = -0.0172$$$

$$$y_{3} = y{\left(t_{3} \right)} = y{\left(\frac{3}{5} \right)} = y_{2} + h f{\left(t_{2} + \frac{h}{2},y_{2} + \frac{h}{2} f{\left(t_{2},y_{2} \right)} \right)} = 0.8172 + \frac{f{\left(\frac{2}{5} + \frac{\frac{1}{5}}{2},0.8172 + \frac{\frac{1}{5}}{2} \left(-0.0172\right) \right)}}{5} = 0.854104$$$

ステップ 4

$$$t_{4} = t_{3} + h = \frac{3}{5} + \frac{1}{5} = \frac{4}{5}$$$

$$$f{\left(t_{3},y_{3} \right)} = f{\left(\frac{3}{5},0.854104 \right)} = 0.345896$$$

$$$y_{4} = y{\left(t_{4} \right)} = y{\left(\frac{4}{5} \right)} = y_{3} + h f{\left(t_{3} + \frac{h}{2},y_{3} + \frac{h}{2} f{\left(t_{3},y_{3} \right)} \right)} = 0.854104 + \frac{f{\left(\frac{3}{5} + \frac{\frac{1}{5}}{2},0.854104 + \frac{\frac{1}{5}}{2} \cdot 0.345896 \right)}}{5} = 0.95636528$$$

ステップ 5

$$$t_{5} = t_{4} + h = \frac{4}{5} + \frac{1}{5} = 1$$$

$$$f{\left(t_{4},y_{4} \right)} = f{\left(\frac{4}{5},0.95636528 \right)} = 0.64363472$$$

$$$y_{5} = y{\left(t_{5} \right)} = y{\left(1 \right)} = y_{4} + h f{\left(t_{4} + \frac{h}{2},y_{4} + \frac{h}{2} f{\left(t_{4},y_{4} \right)} \right)} = 0.95636528 + \frac{f{\left(\frac{4}{5} + \frac{\frac{1}{5}}{2},0.95636528 + \frac{\frac{1}{5}}{2} \cdot 0.64363472 \right)}}{5} = 1.1122195296$$$

$$$y{\left(1 \right)}\approx 1.1122195296$$$A