Pierpont prime

Distribution

Empirically, the Pierpont primes do not seem to be particularly rare or sparsely distributed. There are 36 Pierpont primes less than 10⁶, 59 less than 10⁹, 151 less than 10²⁰, and 789 less than 10¹⁰⁰. There are few restrictions from algebraic factorisations on the Pierpont primes, so there are no requirements like the Mersenne prime condition that the exponent must be prime. Thus, it is expected that among n-digit numbers of the correct form 2 u 3 v + 1 , the fraction of these that are prime should be proportional to 1/n, a similar proportion as the proportion of prime numbers among all n-digit numbers. As there are Θ(n²) numbers of the correct form in this range, there should be Θ(n) Pierpont primes.

Andrew M. Gleason made this reasoning explicit, conjecturing there are infinitely many Pierpont primes, and more specifically that there should be approximately 9n Pierpont primes up to 10ⁿ. According to Gleason's conjecture there are Θ(log N) Pierpont primes smaller than N, as opposed to the smaller conjectural number O(log log N) of Mersenne primes in that range.

Primality testing

When 2 u > 3 v , the primality of 2 u 3 v + 1 can be tested by Proth's theorem. On the other hand, when 2 u < 3 v alternative primality tests for p = 2 u 3 v + 1 are possible based on the factorization of p − 1 as a small even number multiplied by a large power of three.

Pierpont primes found as factors of Fermat numbers

As part of the ongoing worldwide search for factors of Fermat numbers, some Pierpont primes have been announced as factors. The following table gives values of m, k, and n such that

k ⋅ 2 n + 1  divides  2 2 m + 1.

The left-hand side is a Pierpont prime when k is a power of 3; the right-hand side is a Fermat number.

As of 2017, the largest known Pierpont prime is 3 × 2^10829346 + 1, whose primality was discovered by Sai Yik Tang and PrimeGrid in 2014.

Polygon construction

In the mathematics of paper folding, the Huzita axioms define six of the seven types of fold possible. It has been shown that these folds are sufficient to allow the construction of the points that solve any cubic equation. It follows that they allow any regular polygon of N sides to be formed, as long as N > 3 and of the form 2^m3ⁿρ, where ρ is a product of distinct Pierpont primes. This is the same class of regular polygons as those that can be constructed with a compass, straightedge, and angle-trisector. Regular polygons which can be constructed with only compass and straightedge (constructible polygons) are the special case where n = 0 and ρ is a product of distinct Fermat primes, themselves a subset of Pierpont primes.

In 1895, James Pierpont studied the same class of regular polygons; his work is what gives the name to the Pierpont primes. Pierpont generalized compass and straightedge constructions in a different way, by adding the ability to draw conic sections whose coefficients come from previously constructed points. As he showed, the regular N-gons that can be constructed with these operations are the ones such that the totient of N is 3-smooth. Since the totient of a prime is formed by subtracting one from it, the primes N for which Pierpont's construction works are exactly the Pierpont primes. However, Pierpont did not describe the form of the composite numbers with 3-smooth totients. As Gleason later showed, these numbers are exactly the ones of the form 2^m3ⁿρ given above.

The smallest prime that is not a Pierpont (or Fermat) prime is 11; therefore, the hendecagon is the smallest regular polygon that cannot be constructed with compass, straightedge and angle trisector (or origami, or conic sections). All other regular N-gons with 3 ≤ N ≤ 21 can be constructed with compass, straightedge and trisector.

Generalization

A Pierpont prime of the second kind is a prime number of the form 2^u3^v − 1, they are

2, 3, 5, 7, 11, 17, 23, 31, 47, 53, 71, 107, 127, 191, 383, 431, 647, 863, 971, 1151, 2591, 4373, 6143, 6911, 8191, 8747, 13121, 15551, 23327, 27647, 62207, 73727, 131071, 139967, 165887, 294911, 314927, 442367, 472391, 497663, 524287, 786431, 995327, ... (sequence A005105 in the OEIS)

A generalized Pierpont prime is a prime of the form p 1 n 1 ⋅ p 2 n 2 ⋅ p 3 n 3 ⋅ . . . ⋅ p k n k + 1 with k fixed primes {p₁, p₂, p₃, ..., p_k}, p_i < p_j for i < j. A generalized Pierpont prime of the second kind is a prime of the form p 1 n 1 ⋅ p 2 n 2 ⋅ p 3 n 3 ⋅ . . . ⋅ p k n k − 1 with k fixed primes {p₁, p₂, p₃, ..., p_k}, p_i < p_j for i < j. Since all primes greater than 2 are odd, in both kinds p₁ must be 2. The sequences of such primes in OEIS are: