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 ,
,
,且
。那么,最大數(shù)是
,最小數(shù)是
,
看見,經(jīng)過一輪變換, 沒了,我們引入
。
先來(lái)給 定個(gè)界:
-
,即
;
-
,即
;
∴ .
則新一輪的三個(gè)數(shù)變成了 , 9,
,且
。
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 ,
,
,
。
,而
會(huì)被抵消掉,∴最關(guān)鍵的是
:
,而
3?? 假設(shè)第三輪的 3 個(gè)數(shù)是 ,
,
,
。
正如 。但我不能證明序列收斂:
??。
然后我去問了 DeepSeek,它說(shuō):
,
7→1→7→ … ∴百位數(shù)并不嚴(yán)格遞增。真正收斂的是
。
| 2 | → 7 → 6 → 5 | |
| 3 | → 6 → 5 | |
| 4 | → 5 | |
| 5 | → 5 | |
| 6 | → 5 | |
| 7 | → 5 | |
| 8 | → 6 → 5 | |
| 9 | → 7 → 6 → 5 |
只要 ,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 種取法,∵會(huì)重排序,需扣掉位置的影響,即僅有
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
# 123 → 198 → 792 → 693 → 594 → 495 → ... (5)
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# 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:
- 一道小學(xué)二年級(jí)算術(shù)題??引發(fā)的紛爭(zhēng)??;
- 一道小學(xué)四年級(jí)算術(shù)題??引發(fā)的紛爭(zhēng)??;
- 小學(xué)生算術(shù)題??紛爭(zhēng)第三彈??;
- 一道算術(shù)題??背后的算法迭代;
- 小學(xué)生算術(shù)題??紛爭(zhēng)第四彈??;
- 小學(xué)生算術(shù)題??紛爭(zhēng)第五彈??;
- 小學(xué)生算術(shù)題??紛爭(zhēng)第六彈??;
- 小學(xué)生算術(shù)題??紛爭(zhēng)第七彈??;
- 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)??!;
- 小學(xué)生算術(shù)題??紛爭(zhēng)第九彈??:幻方 & 群論;