小學(xué)生算術(shù)題??紛爭(zhēng)第十彈??:Kaprekar constant

從 1, 2, 3, 4, 5, 6, 7, 8, 9 中隨便挑 3 個(gè)數(shù)(如 1, 2, 3),然后用組成的最大三位數(shù)(321)減去最小三位數(shù)(123)……如此往復(fù),最后一定會(huì)得到 495 。

小朋友問我為什么的時(shí)候,我故作鎮(zhèn)定,∵第一反應(yīng)是「難道是 3n + 1 猜想?完蛋????」,但為了讓小朋友們看到「碰到難題,最重要的是「堅(jiān)持????」」,只得硬著頭皮做。結(jié)果,發(fā)現(xiàn)不是 3n + 1 猜想,如釋重負(fù)。證明其實(shí)不難,可能用了一刻鐘吧:

1?? 假設(shè)取的 3 個(gè)數(shù)分別是 a_1, a_2, a_3,且 a_1 < a_2 < a_3。那么,最大數(shù)是 \overline{a_3 a_2 a_1},最小數(shù)是 \overline{a_1 a_2 a_3},

\begin{align} \Delta_1 &= \overline{a_3 a_2 a_1} - \overline{a_1 a_2 a_3} \\ &= (10^2 a_3+ 10 a_2 + a_1) - (10^2 a_1+ 10 a_2 + a_3) \\ &= 10^2 (a_3 - a_1) + (a_1 - a_3) \\ &= 10^2 (a_3 - a_1 -1) + 10 \times 9 + \big(10 - (a_3 - a_1) \big) \\ &= \overline{(a_3 - a_1 -1) 9 \big(10 - (a_3 - a_1) \big)} \end{align}

看見,經(jīng)過一輪變換,a_2 沒了,我們引入 \delta_1 \overset{def}{=} a_3 - a_1 。

先來(lái)給 \delta_1 定個(gè)界:

  • \delta_1 = a_3 - a_1 \ge (a_2 + 1) - a_1 \ge \big( (a_1 + 1) + 1 \big) - a_1 = 2,即 \delta_1 \ge 2 ;
  • \delta_1 = a_3 - a_1 \le 9 - a_1 \le 9 - 1 = 8,即 \delta_1 \le 8 ;

2 \le \delta_1 \le 8 .

則新一輪的三個(gè)數(shù)變成了 \delta_1 -1, 9, 10 - \delta_1,且 (\delta_1 -1) + (10 - \delta_1) = 9

如:321 - 123 = 198,百位上的 1 + 個(gè)位上的 8 = 9

2?? 假設(shè)第二輪的 3 個(gè)數(shù)是 b_1, b_2, b_3,b_1 < b_2 < b_3 = 9 。

b_3 = 9,而 b_2 會(huì)被抵消掉,∴最關(guān)鍵的是 b_1

\begin{align} b_1 &= \min \begin{cases} \delta_1 - 1 \\ 10 - \delta_1 \le 8 \end{cases} \\ &= \begin{cases} \delta_1 - 1 , 若 \delta_1 < \frac{11}{2}, 即 \delta_1 \le 5 \\ 10 - \delta_1 , 若 \delta_1 > \frac{11}{2}, 即 \delta_1 \ge 6 \end{cases} \end{align}

\delta_2 = 9 - b_1 ,而 \Delta_2 = \overline{(\delta_2 - 1) 9 (10 - \delta_2)}

3?? 假設(shè)第三輪的 3 個(gè)數(shù)是 c_1, c_2, c_3,c_1 < c_2 < c_3 = 9 。

c_1 = \min \begin{cases} \delta_2 - 1 \\ 10 - \delta_2 \end{cases}

正如 123 \xrightarrow{321 - 123} 198 \xrightarrow{981 - 189} 792 \xrightarrow{972 - 279} 693 \xrightarrow{963 - 369} 594 \xrightarrow{954 - 459} 495 \xrightarrow{954 - 459} 495 \xrightarrow{954 - 459} \dots 。但我不能證明序列收斂:a_1 < b_1 < c_1 < \dots , a_3 > b_3 > c_3 > \dots ??。

然后我去問了 DeepSeek,它說(shuō):

789 \xrightarrow{987 - 789} 198 \xrightarrow{981 - 189} 792 \xrightarrow{972 - 279} \dots717 → … ∴百位數(shù)并不嚴(yán)格遞增。

真正收斂的是 \{\delta_n\}

\delta_n \delta_{n + 1} \delta_{n + 2} → \dots
2 9 - (\delta_n - 1) = 8 → 7 → 6 → 5
3 9 - (\delta_n - 1) = 7 → 6 → 5
4 9 - (\delta_n - 1) = 6 → 5
5 9 - (\delta_n - 1) = 5 → 5
6 9 - (10 - \delta_n) = 5 → 5
7 9 - (10 - \delta_n) = 6 → 5
8 9 - (10 - \delta_n) = 7 → 6 → 5
9 9 - (10 - \delta_n) = 8 → 7 → 6 → 5

只要 \delta_n = 5,就掉進(jìn)了不動(dòng)點(diǎn)。

得證。把我累出一手汗??。


附程序驗(yàn)證:共有 9 \times 8 \times 7 = 504 種取法,∵會(huì)重排序,需扣掉位置的影響,即僅有 \frac{504}{3 \times 2 \times 1} = 84 種實(shí)質(zhì)不同的取法。

def reorder(ary)
  a = ary.min 
  c = ary.max 
  b = ((ary - [a]) - [c]).first
  # a < b < c
  [a, b, c]
end

def kaprekar(ary, level)
  print "#{ary.join} → "
  a, b, c = reorder(ary)
  delta = c - a
  u = delta - 1
  v = 9
  w = 10 - delta

  if [u, v, w] == ary
    print "... (#{level})\n"
    return
  else
    kaprekar([u, v, w], level + 1)
  end
end

(1..9).to_a.combination(3) do |v|
  v.permutation do |w|
    # p w
    kaprekar(w, 0)
  end
end

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# 754 → 297 → 693 → 594 → 495 → ... (4)
# 458 → 396 → 594 → 495 → ... (3)
# 485 → 396 → 594 → 495 → ... (3)
# 548 → 396 → 594 → 495 → ... (3)
# 584 → 396 → 594 → 495 → ... (3)
# 845 → 396 → 594 → 495 → ... (3)
# 854 → 396 → 594 → 495 → ... (3)
# 459 → 495 → ... (1)
# 495 → ... (0)
# 549 → 495 → ... (1)
# 594 → 495 → ... (1)
# 945 → 495 → ... (1)
# 954 → 495 → ... (1)
# 467 → 297 → 693 → 594 → 495 → ... (4)
# 476 → 297 → 693 → 594 → 495 → ... (4)
# 647 → 297 → 693 → 594 → 495 → ... (4)
# 674 → 297 → 693 → 594 → 495 → ... (4)
# 746 → 297 → 693 → 594 → 495 → ... (4)
# 764 → 297 → 693 → 594 → 495 → ... (4)
# 468 → 396 → 594 → 495 → ... (3)
# 486 → 396 → 594 → 495 → ... (3)
# 648 → 396 → 594 → 495 → ... (3)
# 684 → 396 → 594 → 495 → ... (3)
# 846 → 396 → 594 → 495 → ... (3)
# 864 → 396 → 594 → 495 → ... (3)
# 469 → 495 → ... (1)
# 496 → 495 → ... (1)
# 649 → 495 → ... (1)
# 694 → 495 → ... (1)
# 946 → 495 → ... (1)
# 964 → 495 → ... (1)
# 478 → 396 → 594 → 495 → ... (3)
# 487 → 396 → 594 → 495 → ... (3)
# 748 → 396 → 594 → 495 → ... (3)
# 784 → 396 → 594 → 495 → ... (3)
# 847 → 396 → 594 → 495 → ... (3)
# 874 → 396 → 594 → 495 → ... (3)
# 479 → 495 → ... (1)
# 497 → 495 → ... (1)
# 749 → 495 → ... (1)
# 794 → 495 → ... (1)
# 947 → 495 → ... (1)
# 974 → 495 → ... (1)
# 489 → 495 → ... (1)
# 498 → 495 → ... (1)
# 849 → 495 → ... (1)
# 894 → 495 → ... (1)
# 948 → 495 → ... (1)
# 984 → 495 → ... (1)
# 567 → 198 → 792 → 693 → 594 → 495 → ... (5)
# 576 → 198 → 792 → 693 → 594 → 495 → ... (5)
# 657 → 198 → 792 → 693 → 594 → 495 → ... (5)
# 675 → 198 → 792 → 693 → 594 → 495 → ... (5)
# 756 → 198 → 792 → 693 → 594 → 495 → ... (5)
# 765 → 198 → 792 → 693 → 594 → 495 → ... (5)
# 568 → 297 → 693 → 594 → 495 → ... (4)
# 586 → 297 → 693 → 594 → 495 → ... (4)
# 658 → 297 → 693 → 594 → 495 → ... (4)
# 685 → 297 → 693 → 594 → 495 → ... (4)
# 856 → 297 → 693 → 594 → 495 → ... (4)
# 865 → 297 → 693 → 594 → 495 → ... (4)
# 569 → 396 → 594 → 495 → ... (3)
# 596 → 396 → 594 → 495 → ... (3)
# 659 → 396 → 594 → 495 → ... (3)
# 695 → 396 → 594 → 495 → ... (3)
# 956 → 396 → 594 → 495 → ... (3)
# 965 → 396 → 594 → 495 → ... (3)
# 578 → 297 → 693 → 594 → 495 → ... (4)
# 587 → 297 → 693 → 594 → 495 → ... (4)
# 758 → 297 → 693 → 594 → 495 → ... (4)
# 785 → 297 → 693 → 594 → 495 → ... (4)
# 857 → 297 → 693 → 594 → 495 → ... (4)
# 875 → 297 → 693 → 594 → 495 → ... (4)
# 579 → 396 → 594 → 495 → ... (3)
# 597 → 396 → 594 → 495 → ... (3)
# 759 → 396 → 594 → 495 → ... (3)
# 795 → 396 → 594 → 495 → ... (3)
# 957 → 396 → 594 → 495 → ... (3)
# 975 → 396 → 594 → 495 → ... (3)
# 589 → 396 → 594 → 495 → ... (3)
# 598 → 396 → 594 → 495 → ... (3)
# 859 → 396 → 594 → 495 → ... (3)
# 895 → 396 → 594 → 495 → ... (3)
# 958 → 396 → 594 → 495 → ... (3)
# 985 → 396 → 594 → 495 → ... (3)
# 678 → 198 → 792 → 693 → 594 → 495 → ... (5)
# 687 → 198 → 792 → 693 → 594 → 495 → ... (5)
# 768 → 198 → 792 → 693 → 594 → 495 → ... (5)
# 786 → 198 → 792 → 693 → 594 → 495 → ... (5)
# 867 → 198 → 792 → 693 → 594 → 495 → ... (5)
# 876 → 198 → 792 → 693 → 594 → 495 → ... (5)
# 679 → 297 → 693 → 594 → 495 → ... (4)
# 697 → 297 → 693 → 594 → 495 → ... (4)
# 769 → 297 → 693 → 594 → 495 → ... (4)
# 796 → 297 → 693 → 594 → 495 → ... (4)
# 967 → 297 → 693 → 594 → 495 → ... (4)
# 976 → 297 → 693 → 594 → 495 → ... (4)
# 689 → 297 → 693 → 594 → 495 → ... (4)
# 698 → 297 → 693 → 594 → 495 → ... (4)
# 869 → 297 → 693 → 594 → 495 → ... (4)
# 896 → 297 → 693 → 594 → 495 → ... (4)
# 968 → 297 → 693 → 594 → 495 → ... (4)
# 986 → 297 → 693 → 594 → 495 → ... (4)
# 789 → 198 → 792 → 693 → 594 → 495 → ... (5)
# 798 → 198 → 792 → 693 → 594 → 495 → ... (5)
# 879 → 198 → 792 → 693 → 594 → 495 → ... (5)
# 897 → 198 → 792 → 693 → 594 → 495 → ... (5)
# 978 → 198 → 792 → 693 → 594 → 495 → ... (5)
# 987 → 198 → 792 → 693 → 594 → 495 → ... (5)

Ref:

  1. 一道小學(xué)二年級(jí)算術(shù)題??引發(fā)的紛爭(zhēng)??
  2. 一道小學(xué)四年級(jí)算術(shù)題??引發(fā)的紛爭(zhēng)??;
  3. 小學(xué)生算術(shù)題??紛爭(zhēng)第三彈??;
  4. 一道算術(shù)題??背后的算法迭代
  5. 小學(xué)生算術(shù)題??紛爭(zhēng)第四彈??;
  6. 小學(xué)生算術(shù)題??紛爭(zhēng)第五彈??;
  7. 小學(xué)生算術(shù)題??紛爭(zhēng)第六彈??;
  8. 小學(xué)生算術(shù)題??紛爭(zhēng)第七彈??;
  9. 小學(xué)生算術(shù)題??紛爭(zhēng)第八彈??:我是在 4 堆變 3 堆嗎?我是在教小朋友群同構(gòu)??!
  10. 小學(xué)生算術(shù)題??紛爭(zhēng)第九彈??:幻方 & 群論;
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