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Gilbreath Equation, Gilbreath Polynomials, Upper and Lower Bound for Gilbreath Conjecture

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Abstract
Let $S=\left(s_1, \ldots, s_n\right)$ be a finite sequence of integers. Then $S$ is a Gilbreath sequence of length $n$, $S\in\mathbb{G}_n$, iff $s_1$ is even or odd and $s_2, \ldots, s_n$ are respectively odd or even and $\min\mathbb{K}_{\left(s_1, \ldots, s_m\right)}\leq s_{m+1}\leq\max\mathbb{K}_{\left(s_1, \ldots, s_m\right)}\forall m\in\left[\left.1, n\right)\right.$. This, applied to the order sequence of prime number $P$, defines Gilbreath polynomials and two integer sequences A347924 \cite{oeisA347924} and A347925 \cite{oeisA347925} which are used to prove that Gilbreath conjecture $GC$ is implied by $p_n-2^{n-1}\leqslant\mathcal{P}_{n-1}\left(1\right)$ where $\mathcal{P}_{n-1}\left(1\right)$ is the $n-1$-th Gilbreath polynomial at 1.
Keywords: 
Subject: Computer Science and Mathematics  -   Algebra and Number Theory

1. Introduction to GC

Let the ordered sequence P = 2 , 3 , 5 , 7 , 11 , 13 , 17 , = p 1 , , p n formed by prime numbers, and set
p a b = p a + 1 p a , if b = 1 ; | p a + 1 b 1 p a b 1 | , otherwise
where b [ 1 ; n 1 ] . Gilbreath conjectured that p 1 b = 1 . It is likely that this conjecture is satisfied by many other sequences of integers, so it is necessary to define the general properties of all sequences that satisfy this conjecture. Let G C n denote the G C computed on the sequence p 1 , , p n , then there is an interesting computational proof of G C n by A. M. Odlyzko [3] for n < 10 13 .

2. Properties of Gilbreath sequence

Definition 1. 
Let S = s 1 , s 2 , s 3 , , s n be a finite sequence of integers and
s a b = s a + 1 s a , if b = 1 ; | s a + 1 b 1 s a b 1 | , otherwise
where b [ 1 ; n 1 ] , then S is called Gilbreath sequence iff s 1 b = 1 b .
For example, let S = 2 , 3 , 5 , 9 , 11 , 13 , 17 be a sequence of length n = 7 and Gilbreath triangle of S
s 1 s 2 s 3 s 4 s n 3 s n 2 s n 1 s n
s 1 1 s 2 1 s 3 1 s 4 1 s n 3 1 s n 2 1 s n 1 1
s 1 n 2 s 2 n 2
s 1 n 1
Replacing values gives
2 3 5 9 11 13 17
1 2 4 2 2 4
1 2 2 0 2
1 0 2 2
1 2 0
1 2
1
Let G n denote the set of all Gilbreath sequences of length n and G the set of all Gilbreath sequences. In the previous example the first term of every sequence (except for the original sequence S) is equal to 1, then S G 7 .
Lemma 1. 
Let S = s 1 , , s n G n and S = s 1 , , s n 1 be finite sequences of integers, then S G n 1 .
Proof. 
Consider the Gilbreath triangle of S
s 1 s 2 s 3 s 4 s n 3 s n 2 s n 1 s n
s 1 1 s 2 1 s 3 1 s 4 1 s n 3 1 s n 2 1 s n 1 1
s 1 n 2 s 2 n 2
s 1 n 1
where s 1 1 = = s 1 n 2 = s 1 n 1 = 1 as a consequence of S G n . Removing the last element of each sequence gives
s 1 s 2 s 3 s 4 s n 3 s n 2 s n 1
s 1 1 s 2 1 s 3 1 s 4 1 s n 3 1 s n 2 1
s 1 n 2
which is Gilbreath triangle of S , s 1 1 = = s 1 n 2 = 1 as a consequence of S G n , then S G n 1 . □
Definition 2. 
Let S = s 1 , , s n G n and S = s 1 , , s n , k be finite sequences of integers. Denote with K S the set of integers k such that S G n + 1 .
Gilbreath triangle of S is
s 1 s 2 s 3 s 4 s n 3 s n 2 s n 1 s n
s 1 1 s 2 1 s 3 1 s 4 1 s n 3 1 s n 2 1 s n 1 1
s 1 n 2 s 2 n 2
s 1 n 1
where s 1 1 = = s 1 n 2 = s 1 n 1 = 1 as a consequence of S G n . Gilbreath triangle of S is
s 1 s 2 s 3 s 4 s n 3 s n 2 s n 1 s n k
s 1 1 s 2 1 s 3 1 s 4 1 s n 3 1 s n 2 1 s n 1 1 | s n k |
s 1 n 2 s 2 n 2 | s 3 n 3 | s 4 n 4 | | s n 1 1 | s n k | | | | |
s 1 n 1 | s 2 n 2 | s 3 n 3 | s 4 n 4 | | s n 1 1 | s n k | | | | | |
| s 1 n 1 | s 2 n 2 | s 3 n 3 | s 4 n 4 | | s n 1 1 | s n k | | | | | | |
where s 1 1 = = s 1 n 2 = s 1 n 1 = 1 as a consequence of S G n . If s 1 n = | s 1 n 1 | s 2 n 2 | s 3 n 3 | s 4 n 4 | | s n 1 1 | s n k | | | | | | | = 1 , then S G n + 1 .
Consider the equation
| s 1 n 1 | s 2 n 2 | s 3 n 3 | s 4 n 4 | | s n 1 1 | s n k | | | | | | | = 1
We will refer to equation (3) as Gilbreath equation of S. There are 2 n values of k that satisfy (3), then K S = { k 1 , , k 2 n } is the set of all solutions for k:
k 1 , , 2 n = ± s 1 n 1 ± s 2 n 2 ± s 3 n 3 ± s 4 n 4 ± ± s n 1 1 + s n ± 1
The largest value of k that solves (3) is max K S = s 1 n 1 + s 2 n 2 + s 3 n 3 + s 4 n 4 + + s n 1 1 + s n + 1 and the smallest value is min K S = s 1 n 1 s 2 n 2 s 3 n 3 s 4 n 4 s n 1 1 + s n 1 = 2 s n max K S . A remarkable relation is
max K S + min K S = 2 s n
Lemma 2. 
Let S = s 1 , , s n G n be a finite sequence of integers where s 1 2 Z , then s 2 , , s n 2 Z + 1 .
Proof. 
Let S 1 = s 1 , where s 1 2 Z . From definition 2, S 2 = s 1 , k G 2 if k = s 1 ± 1 2 Z + 1 . Let now the sequence S 3 = s 1 , s 1 ± 1 , k , from definition 2, S 3 G 3 if k = | ± 1 | + s 1 ± 1 ± 1 = 1 + s 1 ± 1 ± 1 . From the previous step, s 1 ± 1 2 Z + 1 . Then 1 + s 1 ± 1 2 Z and 1 + s 1 ± 1 ± 1 2 Z + 1 . By induction, this can be proved for every element of S. If S G n and the first element of S is an even number, then all the other numbers of the sequence will be odd. □
Lemma 3. 
Let S = s 1 , , s n G n be a finite sequence of integers where s 1 2 Z + 1 , then s 2 , , s n 2 Z .
Proof. 
Same argument as lemma 3. □
Lemma 4. 
Let 2 Z + 1 2 ± 1 2 denote the sets 2 Z and 2 Z + 1 and let a finite sequence of integers S = s 1 , , s n G n where s 1 2 Z + 1 2 ± 1 2 . Then s 2 , , s n 2 Z + 1 2 1 2 .
Proof. 
Lemma 2 and lemma 3. □
Lemma 5. 
Let S = s 1 , , s n G n and S = s 1 , , s n , k G n + 1 be finite sequences of integers where s 1 2 Z + 1 2 ± 1 2 . Then
k K S = { x min K S , max K S x 2 Z + 1 2 1 2 } .
Proof. 
Definition 2 and lemma 4. □
An important result regarding equation (4) follows from lemma 5. (4) generates 2 n solutions for a finite sequence s 1 , , s n , k G n + 1 where s 1 , , s n G n . From lemma 5, these solutions are only even or only odd if s 1 is odd of even respectively. Therefore, the number of distinct solutions generated by (4) is 2 n 1 since solutions are divided between even and odd.
Theorem 1. 
Let S = s 1 , , s n G n and S = s 1 , , s n , k be finite sequences of integers, then k K S S G n + 1 .
Proof. 
Suppose that s 1 2 Z + 1 2 ± 1 2 . Prove the right implication first. From definition 2, k min K S , max K S and from lemma 5, k 2 Z + 1 2 1 2 . Then k K S S G n + 1 . Prove the left implication by contradiction. Suppose that S G n + 1 but k K S . Then k { x min K S , max K S x 2 Z + 1 2 1 2 } . From definition 2 and lemma 5 it is not possible to have S G n + 1 if k > max K S k < min K S k 2 Z + 1 2 1 2 . Then it is true k K S S G n + 1 . □
Corollary 1. 
Let S = s 1 , , s n be a finite sequence of integers, then S G n min K s 1 , , s m s m + 1 max K s 1 , , s m m 1 , n .
Proof. 
As a consequence of definition 2 and equation (4), each m-th element of a sequence S must be within the range between the upper and the lower sequences calculated on all the elements prior to the m-th ones. From definition 2 and according to the solution of Gilbreath equation (4), there cannot exist Gilbreath sequence in which the m-th element is larger than max K s 1 , , s m 1 , since max K s 1 , , s m 1 is the maximum value for the m-th element. The same goes for min K s 1 , , s m 1 , since it is the smallest value for the m-th element. □

3. Upper and lower bound sequence for P

Let now introduce the definition of two notable Gilbreath sequences. Let S = s 1 , , s n G n be a finite sequence of integers, from (4), any solutions of the Gilbreath equation cannot be greater than max K S , so the sequence s 1 , , s n , max K S G n + 1 is the upper bound sequence for S. Let now the new sequence S = s 1 , , s n , max K S and its upper bound sequence s 1 , , s n , max K S , max K s 1 , , s n , max K S G n + 2 and so on. Equally, let a finite sequence of integers S = s 1 , , s n , from (4), any value of k cannot be smaller than min K S and the new sequence S = s 1 , , s n , min K S , k will have the lower limit for k = min K s 1 , , s n , min K S and so on. Then, it is possible to introduce the definition of upper bound Gilbreath sequence and lower bound Gilbreath sequence.
Definition 3. 
Let S = s 1 , , s n G n be a finite sequence of integers. Let denote with U S the upper bound Gilbreath sequence for S and with L S the lower bound Gilbreath sequence for S:
U S = u 1 , = s 1 , , s n , max K s 1 , , s n , max K s 1 , , s n , max K s 1 , , s n ,
L S = l 1 , = s 1 , , s n , min K s 1 , , s n , min K s 1 , , s n , min K s 1 , , s n ,
The following recursive definition holds:
u i = s i , if i n ; max K u 1 , , u i 1 , otherwise
and
l i = s i , if i n ; min K u 1 , , u i 1 , otherwise
It is also useful to define a notable sub sequence of U S and L S .
Definition 4. 
Let S G n be a finite sequence of integers and its U S and L S . Let define U ˜ S and L ˜ S as follows:
U ˜ S = u ˜ 1 , = max K S , max K S , max K S ,
L ˜ S = l ˜ 1 , = min K S , min K S , min K S ,
The following recursive definition holds:
u ˜ i = max K S , if i = 1 ; max K S , u ˜ 1 , , u ˜ i 1 , otherwise
l ˜ i = min K S , if i = 1 ; min K S , l ˜ 1 , , l ˜ i 1 , otherwise
From theorem 1, U S G and L S G , while elements of U ˜ S and L ˜ S are all even or all odd, then U ˜ S G and L ˜ S G .
From definition 3, let S = s 1 , then U S = s 1 , s 1 + 1 , s 1 + 3 , , s 1 + 2 n 1 , U ˜ S = s 1 + 1 , s 1 + 3 , , s 1 + 2 n 1 , L S = s 1 , s 1 1 , s 1 3 , , s 1 2 n + 1 and L ˜ S = s 1 1 , s 1 3 , , s 1 2 n + 1 . Table 1 shows some examples of Gilbreath sequences and the closed form for u ˜ n .

4. Gilbreath polynomials

Definition 5. 
Let P = p 1 , , p m be the ordered sequence of first m prime numbers and let P m n be a function such that u ˜ n = 2 m + n 1 + P m n where P m n = a m , 0 + a m , 1 n + + a m , k n k , then P m is called m-th Gilbreath polynomial.
Through simple algebra one can prove that for the ordered sequence of first m prime numbers, P m n are represented in Table 2. This provides important information about sequence A347924 [7] which is the triangle read by rows where row m is the m-th Gilbreath polynomial and column n is the numerator of the coefficient of the k-th degree term. According to Table 2, this sequence contains the integer term of every m-th Gilbreath polynomials. The related sequence A347925 [6] contains the lowest common denominator of m-th Gilbreath polynomial. It is the sequence of denominators of the polynomials in Table 2.

4.1. Relation between Gilbreath polynomials and G C

Gilbreath polynomials are closely related to prime numbers and G C . Let a finite sequence of integers S = s 1 , , s n , from theorem 1 is true the following. The relationship s 2 = s 1 ± 1 must be true, otherwise it would not be true that s 1 1 = 1 . As a consequence of lemma 5, for all elements subsequent to s 1 , the absolute difference of two successive elements must be an integer multiple of 2 so as to maintain the absolute difference of two successive elements as an even value. So, if the first element in the sequence is even, the subsequent elements must be odd and if the first element is odd, the subsequent elements must be even.
Let P = p 1 , p 2 = 2 , 3 G 2 Gilbreath sequence formed by the first two prime numbers. From (5), min K p 1 , p 2 p 2 max K p 1 , p 2 and from theorem 1, p 1 , p 2 , p 2 G 3 . By definition of P, p n > p n 1 . Since min K p 1 , p 2 p 2 , it is certainly true that min K p 1 , p 2 p 3 . The left inequality is proved for n = 3 and it is easy to prove for every n. The proof of min K p 1 , , p n 1 p n is trivial and holds for all prime numbers, hence p n max K p 1 , , p n 1 G C n . Given Gilbreath polynomials in definition 5, max K p 1 , , p n 1 = 2 n 1 + P n 1 1 , then
p n 2 n 1 P n 1 1 G C n
Left side of (6)
p n 2 n 1 P n 1 1
consists of Gilbreath polynomial conjecture whose solution implies G C . Unfortunately, bounds for p n are not enough good to prove (7) however this opens the way for a new approach to the G C [1,2,4,5].

5. Conclusions and future work

Theorem 1, about properties of Gilbreath sequence states that if and only if the first element of a finite sequence of integers s 1 , , s n G n is even or odd, then for any odd or even integer max K S k min K S respectively the sequence s 1 , , s n , k G n + 1 .
Theorem 1 proves equation (6) involving Gilbreath polynomials and equation (7) implies G C . Gilbreath polynomials defined in definition 5 introduce a new interesting tool for the study of the properties of prime numbers, in particular we are interested in the matrix of coefficients of Gilbreath polynomials defined as G = a m , k and a paper on G will be published in the future.

References

  1. Dusart, P. The kth Prime is Greater than k(ln k + ln ln k − 1) for k ≥ 2. Mathematics of Computation 1999, 68, 411–415. [Google Scholar] [CrossRef]
  2. Dusart, P. Estimates of Some Functions Over Primes without R.H. 2010. [Google Scholar]
  3. Odlyzko, A.M. Iterated Absolute Values of Differences of Consecutive Primes. Mathematics of Computation 1993, 61, 373–380. [Google Scholar] [CrossRef]
  4. Rosser, B. The n-th Prime is Greater than n log n. Proceedings of the London Mathematical Society 1939, s2-45, 21–44. [Google Scholar] [CrossRef]
  5. Rosser, B. Explicit Bounds for Some Functions of Prime Numbers. American Journal of Mathematics 1941, 63, 211–232. [Google Scholar] [CrossRef]
  6. Sloane, N.J.A.; Inc., T.O.F. Sequence A347925 from the on-line encyclopedia of integer sequences, 2020.
  7. Sloane, N.J.A.; Inc., T.O.F. Sequence A34794 from the on-line encyclopedia of integer sequences, 2020.
Table 1. Some examples of Gilbreath sequences and their closed form for u ˜ n .
Table 1. Some examples of Gilbreath sequences and their closed form for u ˜ n .
m S G m u ˜ n
2 44 , 45 2 n + 1 + 43
3 21 , 20 , 18 2 n + 2 + 14
4 38 , 39 , 39 , 39 2 n + 3 n 2 5 n + 31
4 6 , 7 , 5 , 3 2 n + 3 n 2 3 n 5
5 28 , 29 , 27 , 25 , 21 2 n + 4 n 2 5 n + 5
6 7 , 8 , 10 , 6 , 6 , 6 2 n + 5 4 n 2 20 n 26
6 13 , 14 , 14 , 14 , 12 , 10 2 n + 5 n 4 12 5 n 3 6 71 n 2 12 115 n 6 22
7 93 , 94 , 94 , 94 , 92 , 92 , 94 2 n + 6 n 4 6 7 n 3 3 77 n 2 6 122 n 3 + 30
Table 2. Gilbreath polynomials for m 16 .
Table 2. Gilbreath polynomials for m 16 .
m P m n
1, 2, 3 1
4 n 2 3 n 1
5 n 2 5 n 5
6 2 n 3 3 5 n 2 55 n 3 19
7 n 4 6 7 n 3 3 77 n 2 6 116 n 3 47
8 n 5 30 2 n 4 3 35 n 3 6 85 n 2 3 1277 n 15 109
9 n 6 180 3 n 5 20 65 n 4 36 155 n 3 12 5327 n 2 90 2579 n 15 233
10 n 7 1260 n 6 36 79 n 5 180 151 n 4 36 2441 n 3 90 1087 n 2 9 36481 n 105 483
11 n 9 181440 n 8 4032 169 n 7 30240 41 n 6 480 8389 n 5 8640 1597 n 4 192 599441 n 3 11340
1202527 n 2 5040 177197 n 252 993
12 n 10 1814400 13 n 9 362880 31 n 8 30240 1123 n 7 60480 20833 n 6 86400 41497 n 5 17280
3375899 n 4 181440 10094093 n 3 90720 12276223 n 2 25200 355399 n 252 2011
13 n 11 19958400 n 10 259200 5 n 9 36288 13 n 8 4320 27841 n 7 604800 46711 n 6 86400
46133 n 5 9072 991007 n 4 25920 50938267 n 3 226800 3525203 n 2 3600 7851061 n 2772 4055
14 n 12 239500800 n 11 2661120 49 n 10 3110400 299 n 9 725760 54871 n 8 7257600 5093 n 7 48384
25465669 n 6 21772800 854669 n 5 80640 60581657 n 4 777600 82179283 n 3 181440 102126421 n 2 51975
15729347 n 2772 8149
15 n 13 3113510400 n 12 29937600 389 n 11 239500800 269 n 10 5443200 7727 n 9 7257600 15781 n 8 907200
4919917 n 7 21772800 13119557 n 6 5443200 58181479 n 5 2721600 424785041 n 4 2721600 4521951163 n 3 4989600
3269429687 n 2 831600 292152089 n 25740 16337
16 n 14 43589145600 17 n 13 6227020800 73 n 12 479001600 2557 n 11 479001600 5777 n 10 43545600 4051 n 9 1612800
11564263 n 8 304819200 20564861 n 7 43545600 107393969 n 6 21772800 471325651 n 5 10886400 18807572041 n 4 59875200
2266391933 n 3 1247400 595484981809 n 2 75675600 818209547 n 36036 32715
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