Carol number

A Carol number is an integer of the form $4^n - 2^{n + 1} - 1$ . An equivalent formula is $(2^n - 1)^2 - 2$ . The first few Carol numbers are: −1, 7, 47, 223, 959, 3967, 16127, 65023, 261119, 1046527 (sequence A093112 in the OEIS).

Carol numbers were first studied by Cletus Emmanuel, who named them after a friend, Carol G. Kirnon.^[1]^[2]

Binary representation

For n > 2, the binary representation of the n-th Carol number is n − 2 consecutive ones, a single zero in the middle, and n + 1 more consecutive ones, or to put it algebraically,

\sum_{i \ne n + 2}^{2n} 2^{i - 1}.

So, for example, 47 is 101111 in binary, 223 is 11011111, etc. The difference between the 2n-th Mersenne number and the n-th Carol number is $2^{n + 1}$ . This gives yet another equivalent expression for Carol numbers, $(2^{2n} - 1) - 2^{n + 1}$ . The difference between the n-th Kynea number and the n-th Carol number is the (n + 2)th power of two.

Primes and modular restrictions

Starting with 7, every third Carol number is a multiple of 7. Thus, for a Carol number to also be a prime number, its index n cannot be of the form 3x + 2 for x > 0. The first few Carol numbers that are also prime are 7, 47, 223, 3967, 16127 (these are listed in Sloane's A091516).

The 7th Carol number and 5th Carol prime, 16127, is also a prime when its digits are reversed, so it is the smallest Carol emirp.^[3] The 12th Carol number and 7th Carol prime, 16769023, is also a Carol emirp.^[4]

As of February 2018, the largest known prime Carol number has index n = 695631, which has 418812 digits.^[5]^[6] It was found by Mark Rodenkirch in July 2016 using the programs CKSieve and PrimeFormGW. It is the 44th Carol prime.

Generalizations

A generalized Carol number base b is defined to be a number of the form (bⁿ−1)² − 2 with n ≥ 1, a generalized Carol number base b can be prime only if b is even, since if b is odd, then all generalized Carol numbers base b are even and thus not prime. A generalized Carol number to base bⁿ is also a generalized Carol number to base b.

Least n ≥ 1 such that ((2b)ⁿ−1)² − 2 is prime are

2, 1, 1, 1, 1, 3, 1, 1, 2, 1, 1, 2, 159, 1, 1, 2, 1, 1, 1, 4, 3, 1, 12, 1, 1, 2, 9, 1, 88, 2, 1, 1, 12, 4, 1, 1, 183, 1, 1, 320, 24, 4, 3, 2, 1, 3, 1, 5, 2, 4, 2, 1, 2, 1, 705, 2, 3, 29, 1, 1, 1, 4836, 20, 1, 135, 1, 4, 1, 6, 1, 15, 3912, 1, 2, 8, 3, 24, 1, 14, 4, 1, 2, 321, 11, 1, 174, 1, 6, 1, 42, 310, 1, 2, 27, 2, 1, 29, 3, 103, 20, ...

b	numbers n ≥ 1 such that (bⁿ−1)² − 2 is prime (these n are checked up to 30000)	OEIS sequence
2	2, 3, 4, 6, 7, 10, 12, 15, 18, 19, 21, 25, 27, 55, 129, 132, 159, 171, 175, 315, 324, 358, 393, 435, 786, 1459, 1707, 2923, 6462, 14289, 39012, 51637, 100224, 108127, 110953, 175749, 185580, 226749, 248949, 253987, 520363, 653490, 688042, 695631, ...	A091515
4	1, 2, 3, 5, 6, 9, 66, 162, 179, 393, 3231, 19506, 50112, 92790, 326745, 344021, ...
6	1, 2, 6, 7, 20, 47, 255, 274, 279, 308, 1162, 2128, 3791, 9028, 9629, 10029, 13202, 38660, 46631, 48257, 117991, ...	A100901
8	1, 2, 4, 5, 6, 7, 9, 43, 44, 53, 57, 105, 108, 131, 145, 262, 569, 2154, 4763, 13004, 33408, 58583, 61860, 75583, 82983, 217830, 231877, ...
10	1, 8, 21, 123, 4299, 6128, 11760, 18884, 40293, ...	A100903
12	3, 29, 51, 7824, 15456, 22614, 28312, 47014, 68835, ...
14	1, 6, 13, 45, 74, 240, 553, 12348, 13659, 50603, ...	A100905
16	1, 3, 33, 81, 9753, 25056, 46395, ...
18	2, 8, 30, 98, 110, 185, 912, 2514, 4074, 10208, 15123, 19395, ...
20	1, 2, 53, 183, 1281, 1300, 8041, 29936, 72820, ...
22	1, 8, 35, 88, 503, 8643, 8743, 14475, ...	A100907
24	2, 27, 92, 4950, 20047, ...
26	159, 879, 4744, 5602, 74387, ...
28	1, 22, 127, 165, 2520, 6492, 6577, 22960, 25528, ...
30	1, 6, 19, 30, 166, 495, 769, 826, 1648, 3993, ...
32	2, 3, 5, 11, 35, 63, 87, 37116, 130698, ...
34	1, 4, 258, ...
36	1, 3, 10, 137, 154, 581, 1064, 4514, 6601, 19330, ...
38	1, 2, 13, 560, 28933, ...
40	4, 15, 39, 138, 2153, 4084, 5639, ...
42	3, 6, 14, 15, 29, 78, 195, 255, 272, 713, 2526, 4852, 10573, ...
44	1, 7, 30, 90, 1288, 1947, 12909, 25786, ...
46	12, 269, 1304, 5172, ...
48	1, 2, 4, 6, 12, 13, 3882, 6123, 15067, 15085, ...
50	1, 3, 4, 9, 31, 66, 115, 430, 1233, 2546, 2674, 6360, 53351, 69033, 69157, ...

As of February 2018, the largest known generalized Carol prime is (2⁶⁹⁵⁶³¹−1)² − 2.

References

↑ Cletus Emmanuel at Prime Pages
↑ Message to Yahoo primenumbers group from Cletus Emmanuel
↑ Prime Curios 16127 at Prime Pages
↑ Prime Curios 16769023 at Prime Pages
↑ Entry for 695631st Carol number at Prime Pages
↑ Carol and Kynea Prime Search by Mark Rodenkirch

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] Cletus Emmanuel at Prime Pages

[2] Message to Yahoo primenumbers group from Cletus Emmanuel

[3] Prime Curios 16127 at Prime Pages

[4] Prime Curios 16769023 at Prime Pages

[5] Entry for 695631st Carol number at Prime Pages

[6] Carol and Kynea Prime Search by Mark Rodenkirch

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 ( $⌊ A 3 n ⌋$ )
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