■三角数,三乗数,五乗数

S(1,n)=Σk=n(n+1)/2

S(2,n)=Σk^2=n(n+1)(2n+1)/6

S(3,n)=Σk^3=n^2(n+1)^2/4

S(4,n)=Σk^4=n(n+1)(2n+1)(3n^2+3n−1)/30

S(5,n)=Σk^5=n^2(n+1)^2(2n^2+2n−1)/12

S(6,n)=Σk^6=n(n+1)(2n+1)(3n^4+6n^3−3n+1)/42

S(6,n)=Σk^7=n^2(n+1)^2(3n^4+6n^3−n^2−4n+2)/24

・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・

ですから,左辺は,sが偶数のときn(n+1)(2n+1)(多項式)/(整数),1以外の奇数のときn^2(n+1)^2(多項式)/(整数)と書くことができます.また,S(s,n)=Σk^sは(s+1)次の多項式になり,最高次数の係数は1/(s+1)です.

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[1]3S(5,n)はいつもS(3,n)で割り切れる.

   S(3,n)はいつもS(1,n)で割り切れる.

  Tn=S(1,n)とおく.

[2]Tn+1^2−Tn^2=(n+1)^3

(証)

{(n+1)(n+2)/2}^2−{n(n+1)/2}^2={(n+2)^2−n^2}{(n+1)/2}^2=4(n+1){(n+1)/2}^2=(n+1)^3

[3]T1+T2+T3=T4

   T5+T6+T7+T8=T9+T10

   T11+T12+T13+T14+T15=T16+T17+T18

   ・・・・・・・・・

(証)4+6+8+・・・+2k=2(2+3+・・・+k)=k(k+1)−2=m,(k+1)(k+2)−2=k^2+3k

k行目の左辺はm+1項目からから始まりk+2項の和,右辺はm+k+3項目から始まりk項の和である.

(左辺)(m+1)(m+2)/2+・・・+(m+k+2)(m+k+3)/2

=Σ(m+i)(m+1+i)/2  (i=1〜k+2)

={Σi^2+(2m+1)Σi+m(m+1)Σ1}/2

=(k+2)(k+3)(2k+5)/12+(2m+1)(k+2)(k+3)/4+m(m+1)(k+2)/2

=(k+2)/12{(k+3)(2k+5)+3(2m+1)(k+3)+6m(m+1)}

=(k+2)/12{(2k^2+11k+15)+3(2k^2+2k−3)(k+3)+6(k^2+k−2)(k^2+k−1)}

=(k+2)/12{6k^4+18k^3+14k^2+2k}

=k(k+2)/12{6k^3+18k^2+14k+2}

(右辺)(m+k+3)(m+k+4)/2+・・・+(m+2k+2)(m+2k+3)/2

=Σ(m+k+2+i)(m+k+3+i)/2  (i=1〜k)

={Σi^2+(2m+2k+5)Σi+(m+k+2)(m+k+3)Σ1}/2

=k(k+1)(2k+1)/12+(2m+2k+5)k(k+1)/4+(m+k+2)(m+k+3)k/2

=k/2{(k+1)(2k+1)/6+(2m+2k+5)(k+1)/2+(m+k+2)(m+k+3)}

=k/12{(k+1)(2k+1)+3(2m+2k+5)(k+1)+6(m+k+2)(m+k+3)}

=k/12{(2k^2+3k+1)+3(2k^2+4k+1)(k+1)+6(k^2+2k)(k^2+2k+1)}

=k/12{6k^4+30k^3+50k^2+30k+4}

=k(k+2)/12{6k^3+18k^2+14k+2}

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