如何證明:質數都是 6 的倍數加減 1

但逆命題不存在 91=6*15+1 ,91不為質數
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1 = 6 * 0 + 1
2
3
5 = 6 * 1 - 1
7 = 6 * 1 + 1
11 = 6 * 2 - 1
13 = 6 * 2 + 1
17 = 6 * 3 - 1
19 = 6 * 3 + 1
23 = 6 * 4 - 1
29 = 6 * 5 - 1
31 = 6 * 5 + 1
37 = 6 * 6 + 1
41 = 6 * 7 - 1
43 = 6 * 7 + 1
47 = 6 * 8 - 1
53 = 6 * 9 - 1
59 = 6 * 10 - 1
61 = 6 * 10 + 1
67 = 6 * 11 + 1
71 = 6 * 12 - 1
73 = 6 * 12 + 1
79 = 6 * 13 + 1
83 = 6 * 14 - 1
89 = 6 * 15 - 1
97 = 6 * 16 + 1