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A257907 After 1, the first differences of A257906 (its d-sequence). 4
1, 2, 3, -2, 4, -3, 5, -1, 7, -9, 8, -4, 9, -8, 10, -5, 15, -19, 11, -10, 12, -7, 17, -18, 16, -13, 19, -17, 21, -16, 26, -29, 23, -21, 25, -23, 27, -26, 28, -27, 29, -25, 33, -35, 31, -22, 40, -45, 35, -34, 36, -33, 39, -41, 37, -31, 43, -42, 44, -47, 41 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

This is sequence (d(n)) generated by the following generic rule, "Rule 3" (see below) with parameters a(1) = 0 and d(1) = 1, while A257906 gives the corresponding sequence a(n).

Rule 3 follows.  For k >= 1, let  A(k) = {a(1), …, a(k)} and D(k) = {d(1), …, d(k)}.  Begin with k = 1 and nonnegative integers a(1) and d(1).

Step 1:  If there is an integer h such that 1 - a(k) < h < 0 and h is not in D(k) and a(k) + h is not in A(k), let d(k+1) be the least such h, let a(k+1) = a(k) + h, replace k by k + 1, and repeat Step 1; otherwise do Step 2.

Step 2:  Let h be the least positive integer not in D(k) such that a(k) - h is not in A(k).  Let a(k+1) = a(k) + h and d(k+1) = h.  Replace k by k+1 and do Step 1.

See A257905 for a guide to related sequences and conjectures.

An informal version of Rule 3 follows:  the sequences "d" and "a" are jointly generated, the "driving idea" idea being that each new term of "d" is obtained by a greedy algorithm.  See A131388 for a similar procedure (Rule 1).

LINKS

Clark Kimberling (first 1000 terms) & Antti Karttunen, Table of n, a(n) for n = 1..10000

EXAMPLE

a(1) = 0, d(1) = 1;

a(2) = 2, d(2) = 2;

a(3) = 5, d(3) = 3;

a(4) = 3, d(4) = -2.

MATHEMATICA

{a, f} = {{0}, {1}}; Do[tmp = {#, # - Last[a]} &[Min[Complement[#, Intersection[a, #]]&[Last[a] + Complement[#, Intersection[f, #]] &[Range[2 - Last[a], -1]]]]];

If[! IntegerQ[tmp[[1]]], tmp = {Last[a] + #, #} &[NestWhile[# + 1 &, 1, ! (! MemberQ[f, #] && ! MemberQ[a, Last[a] - #]) &]]]; AppendTo[a, tmp[[1]]]; AppendTo[f, tmp[[2]]], {120}]; {a, f} (* Peter J. C. Moses, May 14 2015 *)

PROG

(Scheme, with memoization-macro definec)

;; Naive quadratic algorithm, but suffices us for now. Needs also code from A257906:

(definec (A257907 n) (cond ((= 1 n) 1) (else (let ((k (- n 1)) (a_k (A257906 (- n 1)))) (let aloop ((h (+ 1 (- 1 a_k)))) (cond ((zero? h) (let bloop ((h 1)) (cond ((and (not_in_range_of? (- a_k h) A257906 k) (not_in_range_of? h A257907 k)) h) (else (bloop (+ 1 h)))))) ((and (not_in_range_of? (+ a_k h) A257906 k) (not_in_range_of? h A257907 k)) h) (else (aloop (+ 1 h)))))))))

(define (not_in_range_of? k fun upto_n) (let loop ((i 1)) (cond ((> i upto_n) #t) ((= (fun i) k) #f) (else (loop (+ i 1))))))

;; Antti Karttunen, May 20 2015

(Haskell)

import Data.List ((\\))

a257907 n = a257907_list !! (n-1)

a257907_list = 1 : f [0] [1] where

   f xs@(x:_) ds = g [2 - x .. -1] where

     g [] = h : f ((x + h) : xs) (h : ds) where

                  (h:_) = [z | z <- [1..] \\ ds, x - z `notElem` xs]

     g (h:hs) | h `notElem` ds && y `notElem` xs = h : f (y:xs) (h:ds)

              | otherwise = g hs

              where y = x + h

-- Reinhard Zumkeller, Jun 03 2015

CROSSREFS

After initial 1, the first differences of A257906 (the associated a-sequence for this rule).

Cf. A257905.

Sequence in context: A068962 A036380 A241054 * A139094 A159081 A141285

Adjacent sequences:  A257904 A257905 A257906 * A257908 A257909 A257910

KEYWORD

sign,easy

AUTHOR

Clark Kimberling, May 16 2015

STATUS

approved

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Last modified May 22 21:05 EDT 2019. Contains 323503 sequences. (Running on oeis4.)