题面据 Putnam 可核档案整理;中文题意为本站自译,公式请以原始来源为准。 PDF:https://kskedlaya.org/putnam-archive/2017.pdf。
Each of the integers from to is written on a separate card, and then the cards are combined into a deck and shuffled. Three players, , , and , take turns in the order choosing one card at random from the deck. (Each card in the deck is equally likely to be chosen.) After a card is chosen, that card and all higher-numbered cards are removed from the deck, and the remaining cards are reshuffled before the next turn. Play continues until one of the three players wins the game by drawing the card numbered .
Show that for each of the three players, there are arbitrarily large values of for which that player has the highest probability among the three players of winning the game.
从 到 的每个整数都写在一张单独的卡片上,然后将卡片组合成一副牌并洗牌。三名玩家 、 和 按 的顺序轮流从牌堆中随机选择一张牌。 (牌组中的每张牌被选择的可能性相同。)选择一张牌后,该牌和所有编号较大的牌都会从牌组中移除,剩余的牌在下一轮之前重新洗牌。游戏继续进行,直到三名玩家中的一位通过抽出编号为 的卡片赢得游戏。
证明对于三名玩家中的每一位,都有任意大的 值,使得该玩家在三名玩家中赢得比赛的概率最高。
提示 1
先看同余、整除、最大公因数和 p 进赋值。
提示 2
把整数条件转成同余方程、指数比较或下降过程。
提示 3
若要存在性,用构造;若要唯一性,用最小反例、无限下降或模限制。
完整解答
这页先给题面、题型和提示阶梯,完整证明留给读者逐步展开。2017 年 Putnam A5 可先归入数论:第一步把题设翻成对象、条件、目标三行;第二步沿提示寻找不变量、标准构型或关键变形;第三步补齐边界情形,并回到题目原要求核对。
这题适合先独立想一轮再打开提示。不要急着搜索完整解答,先问自己:题面里最硬的限制是哪一句?