■ディオファントス方程式(その14)

[Q]所与の自然数nについて,

[1]1/(a-b)(a-c)+1/(b-a)(b-c)+1/(c-a)(c-b)=n

[2]a/(a-b)(a-c)+b/(b-a)(b-c)+c/(c-a)(c-b)=n

[3]a^2/(a-b)(a-c)+b^2/(b-a)(b-c)+c^2/(c-a)(c-b)=n

[4]a^3/(a-b)(a-c)+b^3/(b-a)(b-c)+c^3/(c-a)(c-b)=n

の自然数解(a,b,c)を求めよ.

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[A]

[1]1/(a-b)(a-c)+1/(b-a)(b-c)+1/(c-a)(c-b)=0

[2]a/(a-b)(a-c)+b/(b-a)(b-c)+c/(c-a)(c-b)=0

[3]a^2/(a-b)(a-c)+b^2/(b-a)(b-c)+c^2/(c-a)(c-b)=1

[4]a^3/(a-b)(a-c)+b^3/(b-a)(b-c)+c^3/(c-a)(c-b)=a+b+c

より,[1][2]はn=0,[3]はn=1,[4]はn=a+b+cのとき,無限に整数解をもちます.

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