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A349462 The a_i coefficients of the standard form decomposition of U(1,X), the Ulam sequence starting with 1, X in the ordered abelian group of linear integer polynomials in X, where the ordering is lexigraphical. 3
0, 1, 2, 4, 4, 5, 7, 10, 11, 13, 16, 17, 19, 20, 22, 23, 25, 26, 28, 31, 34, 38, 40, 40, 43, 44, 46, 49, 52, 55, 59, 62, 64, 68, 70, 76, 79, 85, 88, 97, 100, 103, 106, 109, 112, 115, 118, 121, 124, 137, 139, 142, 145, 148, 151, 154, 157, 163, 166, 172, 176, 178, 181, 184, 187 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

The set U(1,X) can be defined as follows: it is the unique set of linear integer polynomials such that 1,X are the two smallest elements; for any interval [P,Q] where P,Q are linear integer polynomials, U(1,X) intersect [P,Q] has a minimum and a maximum; and finally, an element u > X is in U(1,X) if and only if it is the smallest element larger than the maximum of U(1,X) intersect [1,u - 1] and which can be written as a sum of two distinct elements in U(1,X) in exactly one way.

U(1,X) can be written uniquely as a union of intervals [a_i X + b_i, c_i X + d_i], where c_i X + d_i + 1 < a_{i + 1} X + b_{i + 1} for all indices i. Here, we give just the coefficients a_i.

This set is related to Ulam sequences in an odd way. Let U(1,n) be the sequence of integers starting with 1,n such that every subsequent term is the next smallest element that can be written as the sum of two distinct prior terms in exactly one way. Then, for all integers k, for all sufficiently large n, U(1,n) intersect [1,c_k n + d_k + 1] = eval_n(U(1,X) intersect [1,c_k X + d_k + 1]), where eval_n is the evaluation map sending X to n.

It is conjectured that for all k and n > 3, U(1,n) intersect [1,c_k n + d_k + 1] = eval_n(U(1,X) intersect [1,c_k X + d_k + 1]). At the time of writing, this had been checked up to k = 217529.

U(1,X) can also be defined model-theoretically, as follows. Let *Z denote the hyper-integers. The sequence of sets U(1,n) indexed over positive integers has a unique extension to a sequence of sets indexed over positive hyper-integers. Take any positive non-standard integer H, and consider the corresponding set U(1,H) in this sequence. Then the intersection of U(1,H) with the set of integer polynomials in H will be isomorphic to U(1,X).

REFERENCES

J. Hinman, B. Kuca, A. Schlesinger, and A. Sheydvasser, The Unreasonable Rigidity of Ulam Sequences, J. Number Theory, 194 (2019), 409-425.

A. Sheydvasser, The Ulam Sequence of the Integer Polynomial Ring, J. Integer Seq., accepted.

LINKS

Table of n, a(n) for n=0..64.

J. Hinman, B. Kuca, A. Schlesinger, and A. Sheydvasser, The Unreasonable Rigidity of Ulam Sequences, arXiv:1711.00145 [math.NT], 2017.

EXAMPLE

The first four intervals of U(1,X) are [1,1], [X,2X], [2X + 2, 2X + 2], [4X, 4X] hence the corresponding a_i coefficients are 0,1,2,4.

CROSSREFS

Cf. A349463, A349464, A349465 for the other coefficients. The original Ulam sequence U(1,2) is A002858.

Sequence in context: A182414 A256984 A111138 * A340761 A035625 A219875

Adjacent sequences:  A349459 A349460 A349461 * A349463 A349464 A349465

KEYWORD

nonn

AUTHOR

Arseniy (Senia) Sheydvasser, Nov 19 2021

STATUS

approved

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Last modified September 28 06:00 EDT 2022. Contains 357063 sequences. (Running on oeis4.)