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Pseudoprime

In general, an integer which has a certain property shared by all prime numbers, but is itself not prime, is called a pseudoprime for that particular property.

The most important class of pseudoprimes come from the Fermat's little theorem and hence they are called Fermat pseudoprimes. This theorem states that if p is prime and a is coprime to p, then a^p-1 - 1 is divisible by p. If a number x is not prime, a is coprime to x and x divides a^x-1 - 1, then x is called a pseudoprime to base a. A number x that is a pseudoprime for all values of a that are coprime to x is called a Carmichael number.

The smallest pseudoprime for the base 2 is 341. It is not a prime, since it equals 11 · 31, nevertheless it satisfies Fermats little theorem; 2³⁴¹=2 (mod 341).

There are applications, such as public key cryptography like RSA that need large prime numbers. The usual algorithm to generate prime numbers is to generate a random odd numbers and test them for primality. However, primality tests are slow. If the user does not require the test to be completely accurate (say, he might tolerate a very small chance that a composite number is declared to be prime), there are fast algorithms based on Fermat's Little Theorem. For example, a simple way to check whether a number x is prime, is to pick some random number a, and check whether x divides a ^x - a. If the answer is no, then x cannot be prime, if the answer is yes, x can be called a "probable prime" and the test can be repeated with other values for a. There's a separate article about the Fermat primality test.

Improved versions of this test give strong probable primes. It has been shown by Monier and Rabin that for the improved test that for each a, the chance of catching a false prime is at least 3 in 4.

There are infinitely many pseudoprimes (in fact infinitely many Carmichael numbers), but they are rather rare. There are only 3 pseudo-primes to base 2 below 1000, to a million there are only 245. Pseudoprimes to base 2 are called Poulet numbers or sometimes Sarrus numbers or Fermatians (SIDN A001567) (http://www.research.att.com/cgi-bin/access.cgi/as/njas/sequences/eisA.cgi?Anum=001567). Poulet numbers and Carmichael numbers (in bold) below 10000 are:

n
1	341 = 11 · 31
2	561 = 3 · 11 · 17
3	645 = 3 · 5 · 43
4	1105 = 5 · 13 · 17
5	1387 = 19 · 73
6	1729 = 7 · 13 · 19
7	1905 = 3 · 5 · 127
8	2047 = 23 · 89
9	2465 = 5 · 17 · 29
10	2701 = 37 · 73
11	2821 = 7 · 13 · 31
12	3277 = 29 · 112
13	4033 = 37 · 109
14	4369 = 17 · 257
15	4371 = 3 · 31 · 47
16	4681 = 31 · 151
17	5461 = 43 · 127
18	6601 = 7 · 23 · 41
19	7957 = 73 · 109
20	8321 = 53 · 157
21	8481 = 3 · 11 · 257
22	8911 = 7 · 19 · 67

A Poulet number all of whose divisors d divide 2^d - 2 is called super-Poulet number. There are an infinitely many Poulet numbers which are not super-Poulet Numbers.

The first smallest pseudoprimes for bases a ≤ 200 are given in the following table; the colors mark the number of prime factors.

a	smallest p-p	a	smallest p-p	a	smallest p-p	a	smallest p-p
		51	65 = 5 · 13	101	175 = 5² · 7	151	175 = 5² · 7
2	341 = 11 · 13	52	85 = 5 · 17	102	133 = 7 · 19	152	153 = 3² · 17
3	91 = 7 · 13	53	65 = 5 · 13	103	133 = 7 · 19	153	209 = 11 · 19
4	15 = 3 · 5	54	55 = 5 · 11	104	105 = 3 · 5 · 7	154	155 = 5 · 31
5	124 = 2² · 31	55	63 = 3² · 7	105	451 = 11 · 41	155	231 = 3 · 7 · 11
6	35 = 5 · 7	56	57 = 3 · 19	106	133 = 7 · 19	156	217 = 7 · 31
7	25	57	65 = 5 · 13	107	133 = 7 · 19	157	186 = 2 · 3 · 31
8	9	58	133 = 7 · 19	108	341 = 11 · 31	158	159 = 3 · 53
9	28 = 2² · 7	59	87 = 3 · 29	109	117 = 3² · 13	159	247 = 13 · 19
10	33 = 3 · 11	60	341 = 11 · 31	110	111 = 3 · 37	160	161 = 7 · 23
11	15 = 3 · 5	61	91 = 7 · 13	111	190 = 2 · 5 · 19	161	190=2 · 5 · 19
12	65 = 5 · 13	62	63 = 3² · 7	112	121	162	481 = 13 · 37
13	21 = 3 · 7	63	341 = 11 · 31	113	133 = 7 · 19	163	186 = 2 · 3 · 31
14	15 = 3 · 5	64	65 = 5 · 13	114	115 = 5 · 23	164	165 = 3 · 5 · 11
15	341 = 11 · 13	65	112 = 2⁴ · 7	115	133 = 7 · 19	165	172 = 2² · 43
16	51 = 3 · 17	66	91 = 7 · 13	116	117 = 3² · 13	166	301 = 7 · 43
17	45 = 3² · 5	67	85 = 5 · 17	117	145 = 5 · 29	167	231 = 3 · 7 · 11
18	25	68	69 = 3 · 23	118	119 = 7 · 17	168	169
19	45 = 3² · 5	69	85 = 5 · 17	119	177 = 3 · 59	169	231 = 3 · 7 · 11
20	21 = 3 · 7	70	169	120	121	170	171 = 3² · 19
21	55 = 5 · 11	71	105 = 3 · 5 · 7	121	133 = 7 · 19	171	215 = 5 · 43
22	69 = 3 · 23	72	85 = 5 · 17	122	123 = 3 · 41	172	247 = 13 · 19
23	33 = 3 · 11	73	111 = 3 · 37	123	217 = 7 · 31	173	205 = 5 · 41
24	25	74	75 = 3 · 5²	124	125 = 3³	174	175 = 5² · 7
25	28 = 2² · 7	75	91 = 7 · 13	125	133 = 7 · 19	175	319 = 11 · 19
26	27 = 3³	76	77 = 7 · 11	126	247 = 13 · 19	176	177 = 3 · 59
27	65 = 5 · 13	77	247 = 13 · 19	127	153 = 3² · 17	177	196 = 2² · 7²
28	45 = 3² · 5	78	341 = 11 · 31	128	129 = 3 · 43	178	247 = 13 · 19
29	35 = 5 · 7	79	91 = 7 · 13	129	217 = 7 · 31	179	185 = 5 · 37
30	49	80	81 = 3⁴	130	217 = 7 · 31	180	217 = 7 · 31
31	49	81	85 = 5 · 17	131	143 = 11 · 13	181	195 = 3 · 5 · 13
32	33 = 3 · 11	82	91 = 7 · 13	132	133 = 7 · 19	182	183 = 3 · 61
33	85 = 5 · 17	83	105 = 3 · 5 · 7	133	145 = 5 · 29	183	221 = 13 · 17
34	35 = 5 · 7	84	85 = 5 · 17	134	135 = 3³ · 5	184	185 = 5 · 37
35	51 = 3 · 17	85	129 = 3 · 43	135	221 = 13 · 17	185	217 = 7 · 31
36	91 = 7 · 13	86	87 = 3 · 29	136	265 = 5 · 53	186	187 = 11 · 17
37	45 = 3² · 5	87	91 = 7 · 13	137	148 = 2² · 37	187	217 = 7 · 31
38	39 = 3 · 13	88	91 = 7 · 13	138	259 = 7 · 37	188	189 = 3³ · 7
39	95 = 5 · 19	89	99 = 3² · 11	139	161 = 7 · 23	189	235 = 5 · 47
40	91 = 7 · 13	90	91 = 7 · 13	140	141 = 3 · 47	190	231 = 3 · 7 · 11
41	105 = 3 · 5 · 7	91	115 = 5 · 23	141	355 = 5 · 71	191	217 = 7 · 31
42	205 = 5 · 41	92	93 = 3 · 31	142	143 = 11 · 13	192	217 = 7 · 31
43	77 = 7 · 11	93	301 = 7 · 43	143	213 = 3 · 71	193	276 = 2² · 3 · 23
44	45 = 3² · 5	94	95 = 5 · 19	144	145 = 5 · 29	194	195 = 3 · 5 · 13
45	76 = 2² · 19	95	141 = 3 · 47	145	153 = 3² · 17	195	259 = 7 · 37
46	133 = 7 · 19	96	133 = 7 · 19	146	147 = 3 · 7²	196	205 = 5 · 41
47	65 = 5 · 13	97	105 = 3 · 5 · 7	147	169	197	231 = 3 · 7 · 11
48	49	98	99 = 3² · 11	148	231 = 3 · 7 · 11	198	247 = 13 · 19
49	66 = 2 · 3 · 11	99	145 = 5 · 29	149	175 = 5² · 7	199	225 = 3² · 5²
50	51 = 3 · 17	100	153 = 3² · 17	150	169	200	201 = 3 · 67

See also:

Euler pseudoprime
- base-2 Euler pseudoprimes (SIDN A006970) (http://www.research.att.com/cgi-bin/access.cgi/as/njas/sequences/eisA.cgi?Anum=006970)
Euler-Jacobi pseudoprime
- base-2 Euler-Jacobi pseudoprimes (SIDN A047713) (http://www.research.att.com/cgi-bin/access.cgi/as/njas/sequences/eisA.cgi?Anum=047713)
- base-3 Euler-Jacobi pseudoprimes (SIDN A048950) (http://www.research.att.com/cgi-bin/access.cgi/as/njas/sequences/eisA.cgi?Anum=048950)
Extra strong Lucas pseudoprime[?]
Fibonacci pseudoprime[?]
Frobenius pseudoprime[?]
Lucas pseudoprime[?]
Perrin pseudoprime[?]
Somer-Lucas pseudoprime[?]
Strong Frobenius pseudoprime[?]
Strong Lucas pseudoprime[?]
Strong pseudoprime[?]
- base-2 strong pseudoprimes (SIDN A001262) (http://www.research.att.com/cgi-bin/access.cgi/as/njas/sequences/eisA.cgi?Anum=001262)

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