■サマーヴィルの等面四面体(その563)

【1】F5 in F4

△5を

  P0(m/2,m√5/2,0,m√10/2,h)

  P1(0,0,0,0,0)

  P2(0,0,0,0,5h)

  P3(2m,0,0,0,4h)

  P4(3m/2,m√5/2,m√10/2,0,3h)

  P5(m,m√5,0,0,2h)

としてみる.

  P0P1^2=4m^2+h^2

  P0P2^2=4m^2+16h^2

  P0P3^2=6m^2+9h^2

  P0P4^2=6m^2+4h^2

  P0P5^2=4m^2+h^2

  P1P2^2=25h^2

  P1P3^2=4m^2+16h^2

  P1P4^2=6m^2+9h^2

  P1P5^2=6m^2+4h^2

  P2P3^2=4m^2+h^2

  P2P4^2=6m^2+4h^2

  P2P5^2=6m^2+9h^2

  P3P4^2=4m^2+h^2

  P3P5^2=6m^2+4h^2

  P4P5^2=4m^2+h^2

4m^2+h^2(5)<4m^2+16h^2(2)

6m^2+4h^2(4)<6m^2+9h^2(3)

25h^2(1)

  25h^2=4m^2+h^2=5,h^2=1/5,m^2=6h^2=6/5

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P0を外すと,等間隔ではなくなる点に注意して

  P1P2^2=25h^2

  P1P3^2=4m^2+16h^2

  P1P4^2=6m^2+9h^2

  P1P5^2=6m^2+4h^2

  P2P3^2=4m^2+h^2

  P2P4^2=6m^2+4h^2

  P2P5^2=6m^2+9h^2

  P3P4^2=4m^2+h^2

  P3P5^2=6m^2+4h^2

  P4P5^2=4m^2+h^2

25h^2(1)=4m^2+h^2(3)<4m^2+16h^2(1)

6m^2+4h^2(3)<6m^2+9h^2(2)

F5は

  P1P2=P2P3=P3P4=P4P5=√5

  P1P3=P2P4=P3P5=√8

  P1P4=P2P5=3

  P1P5=√8

であるから,

 25h^2=4m^2+h^2=5,6m^2+4h^2=8

 6m^2+9h^2=9,4m^2+16h^2=8,m^2=6h^2

 h^2=1/5,m^2=6/5

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 また,

  P1(0,0,0,0,0)

  P2(0,0,0,0,5h)

  P3(2m,0,0,0,4h)

  P4(3m/2,m√5/2,m√10/2,0,3h)

  P5(m,m√5,0,0,2h)

のQ(x,y,z,w)がF4を形成すればよいのであるが,

  P2P3^2=4m^2

  P2P4^2=6m^2

  P2P5^2=4m^2

  P3P4^2=4m^2

  P3P5^2=6m^2

  P4P5^2=4m^2

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【2】G5 in G4

 次に外すとなったら,P5だろうか? この場合も等間隔ではなくなる点に注意して

  P1P2^2=25h^2

  P1P3^2=4m^2+16h^2

  P1P4^2=6m^2+9h^2

  P2P3^2=4m^2+h^2

  P2P4^2=6m^2+4h^2

  P3P4^2=4m^2+h^2

 25h^2(1)=4m^2+h^2(2)<4m^2+16h^2(1)

 =6m^2+4h^2(1)<6m^2+9h^2(1)

G5は

  P2P3=P3P4=P4P5=√5

  P2P4=P3P5=√8

  P2P5=3

であるから,

 25h^2=4m^2+h^2=5,4m^2+16h^2=6m^2+4h^2=8

 6m^2+9h^2=9

 h^2=1/5,m^2=6/5はこれを満たす.

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 また,

  P1(0,0,0,0,0)

  P2(0,0,0,0,5h)

  P3(2m,0,0,0,4h)

  P4(3m/2,m√5/2,m√10/2,0,3h)

Q(x,y,z,w)がG4を形成すればよいのであるが,

  P2P4^2=4m^2

  P2P4^2=4m^2

  P3P4^2=6m^2

これが(4,6,6)であれば問題が残ったが(4,4,6)なので(OK).

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