A243700 The lexicographically earliest sequence of distinct terms with a(1) = 1 such that a(n) divides the sum of the first a(n) terms.

1, 3, 2, 5, 9, 7, 8, 13, 15, 11, 14, 16, 26, 24, 41, 29, 18, 28, 20, 30, 22, 32, 25, 33, 43, 45, 31, 37, 50, 52, 54, 56, 58, 35, 87, 38, 55, 67, 40, 60, 72, 44, 63, 77, 79, 47, 70, 49, 121, 88, 53, 129, 94, 96, 98, 100, 59, 89, 105, 107, 62, 158, 113, 65, 102, 68, 103, 189

Offset

1,2

Comments

Once there is a k such that k > n and a(k) > n, n can no longer appear in the sequence, otherwise a(k) would be n. - Franklin T. Adams-Watters, Jun 11 2014

If the sum a(1) + a(2) + ... + a(m) is not divisible by m, then m does not belong to this sequence. Sequence A019444 gives a variant of this sequence, where every positive integer is a term. - Max Alekseyev, Jun 11 2014

Positive integers that do not appear in this sequence form A243864.

Is there any index n > 3 such that a(n) <= n? - Max Alekseyev, Jun 13 2014

From Bill McEachen, May 21 2024: (Start)

Conjecture: For n > 1000, a(n) falls within 1% of one of the following six values. a(n) = n, 1.576385*n, 1.788185*n, 2.576385*n, 2.788185*n, or 3.576285*n, using floor at the low bound and ceiling at the high bound, inclusive.

For example, a(1153) = 1836. This is between floor(1.576385 * 1153 * 0.99) and ceiling(1.576385 * 1153 * 1.01). About 90% of values fall in the three lower slopes. (End)

Links

Max Alekseyev, Table of n, a(n) for n = 1..100000 (first 1100 terms from Jean-Marc Falcoz)

Éric Angelini, a(n) divides the sum of the first a(n) terms of T, posting to the Sequence Fans Mailing List, Jun 11 2014

Hugo Pfoertner, 1.73*10^6 terms, 7z compressed b-file.

Example

1 divides the sum of the first 1 term  (yes:  1/1=1)
3 divides the sum of the first 3 terms (yes:  6/3=2)
2 divides the sum of the first 2 terms (yes:  4/2=2)
5 divides the sum of the first 5 terms (yes: 20/5=4)
9 divides the sum of the first 9 terms (yes: 63/9=7)
7 divides the sum of the first 7 terms (yes: 35/7=5)
8 divides the sum of the first 8 terms (yes: 48/8=6)
...

Prog

(PARI) { printA243700() = my( S=Set(), T=[], s=0, m=1, k); for(n=1, 10^5, k=m; while( ((k==n || setsearch(S, n)) && Mod(s+k, n)) || if(k<n, sum(i=1, k, T[i])%k) || setsearch(S, k), k++); S=setunion(S, [k]); T=concat(T, [k]);  s+=k; if(s%n, S=setunion(S, [n]); ); while(setsearch(S, m), m++); print1(k, ", "); ) }  \\ Max Alekseyev, Jun 13 2014

Crossrefs

Cf. A019444, A243864 (complement), A244010 (partial sums), A244011 (the quotients), A244016 (sorted), A244017, A244018.

Sequence in context: A333398 A257705 A257878 * A193796 A249906 A258930

Adjacent sequences: A243697 A243698 A243699 * A243701 A243702 A243703

Keyword

nonn,nice

Author

N. J. A. Sloane, Jun 12 2014

Extensions

First 1100 terms were computed by Jean-Marc Falcoz.

Status

approved