Woodall number

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In number theory, a Woodall number (Wn) is any natural number of the form

for some natural number n. The first few Woodall numbers are:

1, 7, 23, 63, 159, 383, 895, … (sequence A003261 in the OEIS).

History[edit]

Woodall numbers were first studied by Allan J. C. Cunningham and H. J. Woodall in 1917,[1] inspired by James Cullen's earlier study of the similarly-defined Cullen numbers.

Woodall primes[edit]

Woodall numbers that are also prime numbers are called Woodall primes; the first few exponents n for which the corresponding Woodall numbers Wn are prime are 2, 3, 6, 30, 75, 81, 115, 123, 249, 362, 384, … (sequence A002234 in the OEIS); the Woodall primes themselves begin with 7, 23, 383, 32212254719, … (sequence A050918 in the OEIS).

In 1976 Christopher Hooley showed that almost all Cullen numbers are composite.[2] Hooley's proof was reworked by Hiromi Suyama to show that it works for any sequence of numbers n · 2n+a + b where a and b are integers, and in particular also for Woodall numbers. Nonetheless, it is conjectured that there are infinitely many Woodall primes.[citation needed] As of March 2018, the largest known Woodall prime is 17016602 × 217016602 − 1.[3] It has 5,122,515 digits and was found by Diego Bertolotti in 2018 in the distributed computing project PrimeGrid.[4]

Divisibility properties[edit]

Like Cullen numbers, Woodall numbers have many divisibility properties. For example, if p is a prime number, then p divides

W(p + 1) / 2 if the Jacobi symbol is +1 and
W(3p − 1) / 2 if the Jacobi symbol is −1.[citation needed]

Generalization[edit]

A generalized Woodall number base b is defined to be a number of the form n × bn − 1, where n + 2 > b; if a prime can be written in this form, it is then called a generalized Woodall prime.

Least n such that n × bn - 1 is prime are[5]

3, 2, 1, 1, 8, 1, 2, 1, 10, 2, 2, 1, 2, 1, 2, 167, 2, 1, 12, 1, 2, 2, 29028, 1, 2, 3, 10, 2, 26850, 1, 8, 1, 42, 2, 6, 2, 24, 1, 2, 3, 2, 1, 2, 1, 2, 2, 140, 1, 2, 2, 22, 2, 8, 1, 2064, 2, 468, 6, 2, 1, 362, 1, 2, 2, 6, 3, 26, 1, 2, 3, 20, 1, 2, 1, 28, 2, 38, 5, 3024, 1, 2, 81, 858, 1, 2, 3, 2, 8, 60, 1, 2, 2, 10, 5, 2, 7, 182, 1, 17782, 3, ... (sequence A240235 in the OEIS)
b numbers n such that n × bn - 1 is prime (these n are checked up to 350000) OEIS sequence
1 3, 4, 6, 8, 12, 14, 18, 20, 24, 30, 32, 38, 42, 44, 48, 54, 60, 62, 68, 72, 74, 80, 84, 90, 98, 102, 104, 108, 110, 114, 128, 132, 138, 140, 150, 152, 158, 164, 168, 174, 180, 182, 192, 194, 198, 200, 212, 224, 228, 230, 234, 240, 242, 252, 258, 264, 270, 272, 278, 282, 284, 294, ... (all primes plus 1) A008864
2 2, 3, 6, 30, 75, 81, 115, 123, 249, 362, 384, 462, 512, 751, 822, 5312, 7755, 9531, 12379, 15822, 18885, 22971, 23005, 98726, 143018, 151023, 667071, 1195203, 1268979, 1467763, 2013992, 2367906, 3752948, ... A002234
3 1, 2, 6, 10, 18, 40, 46, 86, 118, 170, 1172, 1698, 1810, 2268, 4338, 18362, 72662, 88392, 94110, 161538, 168660, 292340, 401208, 560750, 1035092, ... A006553
4 1, 2, 3, 5, 8, 14, 23, 63, 107, 132, 428, 530, 1137, 1973, 2000, 7064, 20747, 79574, 113570, 293912, ..., 1993191, ... A086661
5 8, 14, 42, 384, 564, 4256, 6368, 21132, 27180, 96584, 349656, 545082, ... A059676
6 1, 2, 3, 19, 20, 24, 34, 77, 107, 114, 122, 165, 530, 1999, 4359, 11842, 12059, 13802, 22855, 41679, 58185, 145359, 249987, ... A059675
7 2, 18, 68, 84, 3812, 14838, 51582, ... A242200
8 1, 2, 7, 12, 25, 44, 219, 252, 507, 1155, 2259, 2972, 4584, 12422, 13905, 75606, ... A242201
9 10, 58, 264, 1568, 4198, 24500, ... A242202
10 2, 3, 8, 11, 15, 39, 60, 72, 77, 117, 183, 252, 396, 1745, 2843, 4665, 5364, ... A059671
11 2, 8, 252, 1184, 1308, ... A299374
12 1, 6, 43, 175, 821, 910, 1157, 13748, 27032, 71761, 229918, ... A299375
13 2, 6, 563528, ... A299376
14 1, 3, 7, 98, 104, 128, 180, 834, 1633, 8000, 28538, 46605, 131941, 147684, 433734, ... A299377
15 2, 10, 14, 2312, 16718, 26906, 27512, 41260, 45432, 162454, 217606, ... A299378
16 167, 189, 639, ... A299379
17 2, 18, 20, 38, 68, 3122, 3488, 39500, ... A299380
18 1, 2, 6, 8, 10, 28, 30, 39, 45, 112, 348, 380, 458, 585, 17559, 38751, 43346, 46984, 92711, ... A299381
19 12, 410, 33890, 91850, 146478, 189620, 280524, ... A299382
20 1, 18, 44, 60, 80, 123, 429, 1166, 2065, 8774, 35340, 42968, 50312, 210129, ... A299383
21 2, 18, 200, 282, 294, 1174, 2492, 4348, ...
22 2, 5, 140, 158, 263, 795, 992, 341351, ...
23 29028, ...
24 1, 2, 5, 12, 124, 1483, 22075, 29673, 64593, ...
25 2, 68, 104, 450, ...
26 3, 8, 79, 132, 243, 373, 720, 1818, 11904, 134778, ...
27 10, 18, 20, 2420, 6638, 11368, 14040, 103444, ...
28 2, 5, 6, 12, 20, 47, 71, 624, 1149, 2399, 8048, 30650, 39161, ...
29 26850, 237438, 272970, ...
30 1, 63, 331, 366, 1461, 3493, 4002, 5940, 13572, 34992, 182461, 201038, ...

As of September 2017, the largest known generalized Woodall prime is 1993191×41993191 − 1.

See also[edit]

References[edit]

  1. ^ Cunningham, A. J. C; Woodall, H. J. (1917), "Factorisation of and ", Messenger of Mathematics, 47: 1–38 .
  2. ^ Everest, Graham; van der Poorten, Alf; Shparlinski, Igor; Ward, Thomas (2003). Recurrence sequences. Mathematical Surveys and Monographs. 104. Providence, RI: American Mathematical Society. p. 94. ISBN 0-8218-3387-1. Zbl 1033.11006. 
  3. ^ "The Prime Database: 8508301*2^17016603-1", Chris Caldwell's The Largest Known Primes Database, retrieved March 24, 2018 
  4. ^ PrimeGrid, Anouncement of 17016602*2^17016602 - 1 (PDF), retrieved April 1, 2018 
  5. ^ List of generalized Woodall primes base 3 to 10000

Further reading[edit]

External links[edit]