Williams number

In number theory, a Williams number base b is a natural number of the form $(b-1)\cdot b^{n}-1$ for integers b ≥ 2 and n ≥ 1.[1] The Williams numbers base 2 are exactly the Mersenne numbers.

Williams prime

A Williams prime is a Williams number that is prime. They were considered by Hugh C. Williams.[2]

Least n ≥ 1 such that (b−1)·bⁿ − 1 is prime are: (start with b = 2)

2, 1, 1, 1, 1, 1, 3, 1, 1, 1, 1, 2, 1, 14, 1, 1, 2, 6, 1, 1, 1, 55, 12, 1, 133, 1, 20, 1, 2, 1, 1, 2, 15, 3, 1, 7, 136211, 1, 1, 7, 1, 7, 7, 1, 1, 1, 2, 1, 25, 1, 5, 3, 1, 1, 1, 1, 2, 3, 1, 1, 899, 3, 11, 1, 1, 1, 63, 1, 13, 1, 25, 8, 3, 2, 7, 1, 44, 2, 11, 3, 81, 21495, 1, 2, 1, 1, 3, 25, 1, 519, 77, 476, 1, 1, 2, 1, 4983, 2, 2, ...

b	numbers n ≥ 1 such that (b−1)×bⁿ−1 is prime (these n are checked up to 25000)	OEIS sequence
2	2, 3, 5, 7, 13, 17, 19, 31, 61, 89, 107, 127, 521, 607, 1279, 2203, 2281, 3217, 4253, 4423, 9689, 9941, 11213, 19937, 21701, 23209, 44497, 86243, 110503, 132049, 216091, 756839, 859433, 1257787, 1398269, 2976221, 3021377, 6972593, 13466917, 20996011, 24036583, 25964951, 30402457, 32582657, 37156667, 42643801, 43112609, 57885161, 74207281, 77232917, 82589933, ...	A000043
3	1, 2, 3, 7, 8, 12, 20, 23, 27, 35, 56, 62, 68, 131, 222, 384, 387, 579, 644, 1772, 3751, 5270, 6335, 8544, 9204, 12312, 18806, 21114, 49340, 75551, 90012, 128295, 143552, 147488, 1010743, 1063844, 1360104, ...	A003307
4	1, 2, 3, 9, 17, 19, 32, 38, 47, 103, 108, 153, 162, 229, 235, 637, 1638, 2102, 2567, 6338, 7449, 12845, 20814, 40165, 61815, 77965, 117380, 207420, 351019, 496350, 600523, 1156367, 2117707, 5742009, 5865925, 5947859, ...	A272057
5	1, 3, 9, 13, 15, 25, 39, 69, 165, 171, 209, 339, 2033, 6583, 15393, 282989, 498483, 504221, 754611, 864751, ...	A046865
6	1, 2, 6, 7, 11, 23, 33, 48, 68, 79, 116, 151, 205, 1016, 1332, 1448, 3481, 3566, 3665, 11233, 13363, 29166, 44358, 58530, 191706, ...	A079906
7	1, 2, 7, 18, 55, 69, 87, 119, 141, 189, 249, 354, 1586, 2135, 2865, 2930, 4214, 7167, 67485, 74402, 79326, ...	A046866
8	3, 7, 15, 59, 6127, 8703, 11619, 23403, 124299, ...	A268061
9	1, 2, 5, 25, 85, 92, 97, 649, 2017, 2978, 3577, 4985, 17978, 21365, 66002, 95305, 142199, ...	A268356
10	1, 3, 7, 19, 29, 37, 93, 935, 8415, 9631, 11143, 41475, 41917, 48051, 107663, 212903, 223871, 260253, 364521, 383643, 1009567, ...	A056725
11	1, 3, 37, 119, 255, 355, 371, 497, 1759, 34863, 50719, 147709, 263893, ...	A046867
12	1, 2, 21, 25, 33, 54, 78, 235, 1566, 2273, 2310, 4121, 7775, 42249, 105974, 138961, ...	A079907
13	2, 7, 11, 36, 164, 216, 302, 311, 455, 738, 1107, 2244, 3326, 4878, 8067, 46466, ...	A297348
14	1, 3, 5, 27, 35, 165, 209, 2351, 11277, 21807, 25453, 52443, ...	A273523
15	14, 33, 43, 20885, ...
16	1, 20, 29, 43, 56, 251, 25985, 27031, 142195, 164066, ...
17	1, 3, 71, 139, 265, 793, 1729, 18069, ...
18	2, 6, 26, 79, 91, 96, 416, 554, 1910, 4968, ...
19	6, 9, 20, 43, 174, 273, 428, 1388, ...
20	1, 219, 223, 3659, ...
21	1, 2, 7, 24, 31, 60, 230, 307, 750, 1131, 1665, 1827, 8673, ...
22	1, 2, 5, 19, 141, 302, 337, 4746, 5759, 16530, ...
23	55, 103, 115, 131, 535, 1183, 9683, ...
24	12, 18, 63, 153, 221, 1256, 13116, 15593, ...
25	1, 5, 7, 30, 75, 371, 383, 609, 819, 855, 7130, 7827, 9368, ...
26	133, 205, 215, 1649, ...
27	1, 3, 5, 13, 15, 31, 55, 151, 259, 479, 734, 1775, 2078, 6159, 6393, 9013, ...
28	20, 1091, 5747, 6770, ...
29	1, 7, 11, 57, 69, 235, 16487, ...
30	2, 83, 566, 938, 1934, 2323, 3032, 7889, 8353, 9899, 11785, ...

As of September 2018, the largest known Williams prime base 3 is 2×3^1360104−1.[3]

Generalization

A Williams number of the second kind base b is a natural number of the form $(b-1)\cdot b^{n}+1$ for integers b ≥ 2 and n ≥ 1, a Williams prime of the second kind is a Williams number of the second kind that is prime. The Williams primes of the second kind base 2 are exactly the Fermat primes.

Least n ≥ 1 such that (b−1)·bⁿ + 1 is prime are: (start with b = 2)

1, 1, 1, 2, 1, 1, 2, 1, 3, 10, 3, 1, 2, 1, 1, 4, 1, 29, 14, 1, 1, 14, 2, 1, 2, 4, 1, 2, 4, 5, 12, 2, 1, 2, 2, 9, 16, 1, 2, 80, 1, 2, 4, 2, 3, 16, 2, 2, 2, 1, 15, 960, 15, 1, 4, 3, 1, 14, 1, 6, 20, 1, 3, 946, 6, 1, 18, 10, 1, 4, 1, 5, 42, 4, 1, 828, 1, 1, 2, 1, 12, 2, 6, 4, 30, 3, 3022, 2, 1, 1, 8, 2, 4, 4, 2, 11, 8, 2, 1, ... (sequence A305531 in the OEIS)

b	numbers n ≥ 1 such that (b−1)×bⁿ+1 is prime (these n are checked up to 25000)	OEIS sequence
2	1, 2, 4, 8, 16, ...
3	1, 2, 4, 5, 6, 9, 16, 17, 30, 54, 57, 60, 65, 132, 180, 320, 696, 782, 822, 897, 1252, 1454, 4217, 5480, 6225, 7842, 12096, 13782, 17720, 43956, 64822, 82780, 105106, 152529, 165896, 191814, 529680, 1074726, 1086112, 1175232, ...	A003306
4	1, 3, 4, 6, 9, 15, 18, 33, 138, 204, 219, 267, 1104, 1408, 1584, 1956, 17175, 21147, 24075, 27396, 27591, 40095, 354984, 400989, 916248, 1145805, 2541153, 5414673, ...	A326655
5	2, 6, 18, 50, 290, 2582, 20462, 23870, 26342, 31938, 38122, 65034, 70130, 245538, ...	A204322
6	1, 2, 4, 17, 136, 147, 203, 590, 754, 964, 970, 1847, 2031, 2727, 2871, 5442, 7035, 7266, 11230, 23307, 27795, 34152, 42614, 127206, 133086, ...	A247260
7	1, 4, 9, 99, 412, 2633, 5093, 5632, 28233, 36780, 47084, 53572, ...	A245241
8	2, 40, 58, 60, 130, 144, 752, 7462, 18162, 69028, 187272, 268178, 270410, 497284, 713304, 722600, 1005254, ...	A269544
9	1, 4, 5, 11, 26, 29, 38, 65, 166, 490, 641, 2300, 9440, 44741, 65296, 161930, ...	A056799
10	3, 4, 5, 9, 22, 27, 36, 57, 62, 78, 201, 537, 696, 790, 905, 1038, 66886, 70500, 91836, 100613, 127240, ...	A056797
11	10, 24, 864, 2440, 9438, 68272, 148602, ...	A057462
12	3, 4, 35, 119, 476, 507, 6471, 13319, 31799, ...	A251259
13	1, 2, 4, 21, 34, 48, 53, 160, 198, 417, 773, 1220, 5361, 6138, 15557, 18098, ...
14	2, 40, 402, 1070, 6840, ...
15	1, 3, 4, 9, 11, 14, 23, 122, 141, 591, 2115, 2398, 2783, 3692, 3748, 10996, 16504, ...
16	1, 3, 11, 12, 28, 42, 225, 702, 782, 972, 1701, 1848, 8556, 8565, 10847, 12111, 75122, 183600, 307400, 342107, 416936, ...
17	4, 20, 320, 736, 2388, 3344, 8140, ...
18	1, 6, 9, 12, 22, 30, 102, 154, 600, ...
19	29, 32, 59, 65, 303, 1697, 5358, 9048, ...
20	14, 18, 20, 38, 108, 150, 640, 8244, ...
21	1, 2, 3, 4, 12, 17, 38, 54, 56, 123, 165, 876, 1110, 1178, 2465, 3738, 7092, 8756, 15537, 19254, 24712, ...
22	1, 9, 53, 261, 1491, 2120, 2592, 6665, 9460, 15412, 24449, ...
23	14, 62, 84, 8322, 9396, 10496, 24936, ...
24	2, 4, 9, 42, 47, 54, 89, 102, 118, 269, 273, 316, 698, 1872, 2126, 22272, ...
25	1, 4, 162, 1359, 2620, ...
26	2, 18, 100, 1178, 1196, 16644, ...
27	4, 5, 167, 408, 416, 701, 707, 1811, 3268, 3508, 7020, 7623, 16449, ...
28	1, 2, 136, 154, 524, 1234, 2150, 2368, 7222, 10082, 14510, 16928, ...
29	2, 4, 6, 44, 334, 24714, ...
30	4, 5, 9, 18, 71, 124, 165, 172, 888, 2218, 3852, 17871, 23262, ...

As of September 2018, the largest known Williams prime of the second kind base 3 is 2×3^1175232+1.[4]

A Williams number of the third kind base b is a natural number of the form $(b+1)\cdot b^{n}-1$ for integers b ≥ 2 and n ≥ 1, the Williams number of the third kind base 2 are exactly the Thabit numbers. A Williams prime of the third kind is a Williams number of the third kind that is prime.

A Williams number of the fourth kind base b is a natural number of the form $(b+1)\cdot b^{n}+1$ for integers b ≥ 2 and n ≥ 1, a Williams prime of the fourth kind is a Williams number of the fourth kind that is prime, such primes do not exist for $b\equiv 1{\bmod {3}}$ .

b	numbers n such that $(b+1)\cdot b^{n}-1$ is prime	numbers n such that $(b+1)\cdot b^{n}+1$ is prime
2	OEIS: A002235	OEIS: A002253
3	OEIS: A005540	OEIS: A005537
5	OEIS: A257790	OEIS: A143279
10	OEIS: A111391	(not exist)

It is conjectured that for every b ≥ 2, there are infinitely many Williams primes of the first kind (the original Williams primes) base b, infinitely many Williams primes of the second kind base b, and infinitely many Williams primes of the third kind base b. Besides, if b is not = 1 mod 3, then there are infinitely many Williams primes of the fourth kind base b.

Dual form

If we let n take negative values, and choose the numerator of the numbers, then we get these numbers:

Dual Williams numbers of the first kind base b: numbers of the form $b^{n}-(b-1)$ with b ≥ 2 and n ≥ 1.

Dual Williams numbers of the second kind base b: numbers of the form $b^{n}+(b-1)$ with b ≥ 2 and n ≥ 1.

Dual Williams numbers of the third kind base b: numbers of the form $b^{n}-(b+1)$ with b ≥ 2 and n ≥ 1.

Dual Williams numbers of the fourth kind base b: numbers of the form $b^{n}+(b+1)$ with b ≥ 2 and n ≥ 1. (not exist when b = 1 mod 3)

Unlike the original Williams primes of each kind, some large dual Williams primes of each kind are only probable primes, since for these primes N, neither N−1 not N+1 can be trivially written into a product.

b	numbers n such that $b^{n}-(b-1)$ is (probable) prime (dual Williams primes of the first kind)	numbers n such that $b^{n}+(b-1)$ is (probable) prime (dual Williams primes of the second kind)	numbers n such that $b^{n}-(b+1)$ is (probable) prime (dual Williams primes of the third kind)	numbers n such that $b^{n}+(b+1)$ is (probable) prime (dual Williams primes of the fourth kind)
2	OEIS: A000043	(see Fermat prime)	OEIS: A050414	OEIS: A057732
3	OEIS: A014224	OEIS: A051783	OEIS: A058959	OEIS: A058958
4	OEIS: A059266	OEIS: A089437	OEIS: A217348	(not exist)
5	OEIS: A059613	OEIS: A124621	OEIS: A165701	OEIS: A089142
6	OEIS: A059614	OEIS: A145106	OEIS: A217352	OEIS: A217351
7	OEIS: A191469	OEIS: A217130	OEIS: A217131	(not exist)
8	OEIS: A217380	OEIS: A217381	OEIS: A217383	OEIS: A217382
9	OEIS: A177093	OEIS: A217385	OEIS: A217493	OEIS: A217492
10	OEIS: A095714	OEIS: A088275	OEIS: A092767	(not exist)

(for the smallest dual Williams primes of the 1st, 2nd and 3rd kinds base b, see OEIS: A113516, OEIS: A076845 and OEIS: A178250)

It is conjectured that for every b ≥ 2, there are infinitely many dual Williams primes of the first kind (the original Williams primes) base b, infinitely many dual Williams primes of the second kind base b, and infinitely many dual Williams primes of the third kind base b. Besides, if b is not = 1 mod 3, then there are infinitely many dual Williams primes of the fourth kind base b.

gollark: See, your intuitive self-evaluation isn't actually right and you should try and remember that maybe.

gollark: Great!

gollark: So you should simply not do that.

gollark: You are an entirely okay person™ who also has fairly useful skills™ in programming and whatever.

gollark: Anyway, kit, you CLEARLY judge yourself somewhat unfairly because something something mental health.

References

Williams primes
See Table 1 in the last page of the paper: Williams, H. C. (1981). "The primality of certain integers of the form 2 A rⁿ – 1". Acta Arith. 39: 7–17. doi:10.4064/aa-39-1-7-17.
The Prime Database: 2·3^1360104 − 1
The Prime Database: 2·3^1175232 + 1

External links

This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.

[1] Williams primes

[2] See Table 1 in the last page of the paper: Williams, H. C. (1981). "The primality of certain integers of the form 2 A rⁿ – 1". Acta Arith. 39: 7–17. doi:10.4064/aa-39-1-7-17.

[3] The Prime Database: 2·3^1360104 − 1

[4] The Prime Database: 2·3^1175232 + 1

Prime number classes
By formula	Fermat ( $2 2 n + 1$ ) Mersenne ( $2 p - 1$ ) Double Mersenne ( $2 2 p -1 - 1$ ) Wagstaff $(2 p + 1)/3$ Proth ( $k \cdot2 n + 1$ ) Factorial ( $n! \pm 1$ ) Primorial ( $p n # \pm 1$ ) Euclid ( $p n # + 1$ ) Pythagorean ( $4 n + 1$ ) Pierpont ( $2 m \cdot3 n + 1$ ) Quartan ( $x 4 + y 4$ ) Solinas ( $2 m \pm 2 n \pm 1$ ) Cullen ( $n \cdot2 n + 1$ ) Woodall ( $n \cdot2 n - 1$ ) Cuban ( $x 3 - y 3)/(x - y$ ) Carol $(2 n - 1) 2 - 2$ Kynea $(2 n + 1) 2 - 2$ Leyland ( $x y + y x$ ) Thabit ( $3\cdot2 n - 1$ ) Williams ( $(b -1)\cdot b n - 1$ ) Mills ( $⌊ A 3 n ⌋$ )
By integer sequence	Fibonacci Lucas Pell Newman–Shanks–Williams Perrin Partitions Bell Motzkin
By property	Wieferich (pair) Wall–Sun–Sun Wolstenholme Wilson Lucky Fortunate Ramanujan Pillai Regular Strong Stern Supersingular (elliptic curve) Supersingular (moonshine theory) Good Super Higgs Highly cototient
Base-dependent	Happy Dihedral Palindromic Emirp Repunit $(10 n - 1)/9$ Permutable Circular Truncatable Strobogrammatic Minimal Weakly Full reptend Unique Primeval Self Smarandache–Wellin Tetradic
Patterns	Twin ( $p, p + 2$ ) Bi-twin chain ( $n - 1, n + 1, 2 n - 1, 2 n + 1, \dots$ ) Triplet ( $p, p + 2 or p + 4, p + 6$ ) Quadruplet ( $p, p + 2, p + 6, p + 8$ ) k−Tuple Cousin ( $p, p + 4$ ) Sexy ( $p, p + 6$ ) Chen Sophie Germain/Safe ( $p, 2 p + 1$ ) Cunningham ( $p, 2 p \pm 1, 4 p \pm 3, 8 p \pm 7, ...$ ) Arithmetic progression ( $p + a\cdotn, n = 0, 1, 2, 3, ...$ ) Balanced ( $consecutive p - n, p, p + n$ )
By size	Titanic (1,000+ digits) Gigantic (10,000+ digits) Mega (1,000,000+ digits) Largest known
Complex numbers	Eisenstein prime Gaussian prime
Composite numbers	Pseudoprime Catalan Elliptic Euler Euler–Jacobi Fermat Frobenius Lucas Somer–Lucas Strong Carmichael number Almost prime Semiprime Interprime Pernicious
Related topics	Probable prime Industrial-grade prime Illegal prime Formula for primes Prime gap
First 60 primes	2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 101 103 107 109 113 127 131 137 139 149 151 157 163 167 173 179 181 191 193 197 199 211 223 227 229 233 239 241 251 257 263 269 271 277 281
List of prime numbers