[離散] 生成函數

[mqazz1 (無法顯示)]

Find the generating function for the number of integer solutions to

x1 + x2 + x3 + x4 = r ,  with 1 <= x1 <= x2 <= x3 <= x4


請問這題要怎麼解?

謝謝

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◆ From: 192.192.13.101
推 XII         :4y1+3y2+2y3+y4=r,y1>=1;y2,y3,y4>=0
→ XII         :x^4/{(1-x^4)(1-x^3)(1-x^2)(1-x)}
→ mqazz1      :請問為什麼分子有x^4 @@?