■サマーヴィルの等面四面体(その477)

[x  1  0  0    0] [x 1 0 0    0]

[1 2x  1  0     ] [1/2 x 1/2 0     ]

[0  1 2x  1     ]=2^n-1[0 1/2 x 1/2     ]

[0  0  1 2x     ]    [0 0 1/2 x     ]

[          2x  1]    [      1/2 x 1/2]

[0          1 2x]    [0       1/2 x ]

=cosnθ=Tn(x)

は第1種チェビシェフ多項式の行列式表示となる.

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 第1行目が[x 1/√2 0 0    0]にならないことが気になるが,(その472)と同様に計算すると

x[x 1 0 0   0 ]-√cw [x 1 0 0   0 ]

[1/2 x 1/2 0     ] [1/2 x 1/2 0     ]

[0 1/2 x 1/2    ] [0 1/2 x 1/2    ]

[0 0 1/2 x     ] [0 0 1/2 x     ]

[        x 1/2 ] [        x 0 ]

[0       1/2 x ] [0       1/2 √cw]

   (n-1行)            (n-1行)

x[x 1 0 0   0 ]-cw [x 1 0 0  ]

[1/2 x 1/2 0     ] [1/2 x 1/2 0   ]

[0 1/2 x 1/2    ] [0 1/2 x 1/2 ]

[0 0 1/2 x     ] [0 0 1/2 x 1/2]

[        x 1/2 ] [0     1/2 x ]

[0       1/2 x ]

   (n-1行)            (n-2行)

2^n-1をかけると

x2^n-1[x 1 0 0   0 ]-2cw2^n-2[x 1 0 0  ]

   [1/2 x 1/2 0     ]      [1/2 x 1/2 0   ]

   [0 1/2 x 1/2    ]     [0 1/2 x 1/2 ]

   [0 0 1/2 x     ]     [0 0 1/2 x 1/2]

   [        x 1/2 ]     [0     1/2 x ]

   [0       1/2 x ]

=xcos(n-1)θ-2cwcos(n-2)θ

=cosθcos(n-1)θ-2cwcos(n-2)θ

=cosθcos(n-1)θ-2cw{cos(n-1)θcosθ+sin(n-1)θsinθ}

=cosθcos(n-1)θ-2cos^2π/w{cos(n-1)θcosθ+sin(n-1)θsinθ}

=(1-2cos^2π/w)cosθcos(n-1)θ-2cos^2π/w・sin(n-1)θsinθ

=(1-2cos^2π/w)cosθcos(n-1)θ-2cos^2π/w・sin(n-1)θsinθ+sin(n-1)θsinθ-sin(n-1)θsinθ

=-cos2π/w・cosθcos(n-1)θ-(1+cos2π/w)・sin(n-1)θsinθ}

=-cosθcos(n-1)θ{cos2π/w+(1+cos2π/w)・tan(n-1)θtanθ}=0

-1=(1+sec2π/w)tan(n-1)θtanθ

ここに誤りがあっった.

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