login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A283681 Unique sequence with a(1)=1, a(2)=2, representing an array read by antidiagonals in which the i-th row is this sequence itself multiplied by i. 5
1, 2, 2, 2, 4, 3, 2, 4, 6, 4, 4, 4, 6, 8, 5, 3, 8, 6, 8, 10, 6, 2, 6, 12, 8, 10, 12, 7, 4, 4, 9, 16, 10, 12, 14, 8, 6, 8, 6, 12, 20, 12, 14, 16, 9, 4, 12, 12, 8, 15, 24, 14, 16, 18, 10, 4, 8, 18, 16, 10, 18, 28, 16, 18, 20, 11, 4, 8, 12, 24, 20, 12, 21, 32, 18, 20, 22, 12, 6, 8 (list; table; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Any integer greater than 1 appears infinitely many times.

In particular, any n appears at the position (n^2 + n)/2. For prime n > 2, this is its first appearance; for composite n, it is not the first.

2 appears at the positions 2, 3, 4, 7, 22, 232, 26797, ... (A007501(n) + 1).

When the sequence is considered as an array, any prime n appears only in the first row (infinitely many times) and in the first column (once).

LINKS

Ivan Neretin, Table of n, a(n) for n = 1..26796

FORMULA

a((n^2+n)/2)=n.

EXAMPLE

The sequence begins: 1, 2, 2, 2, 4, 3, 2, 4, 6, 4, ...

It represents a rectangular array read by downward antidiagonals. The first row of the array is this very sequence itself. The second row is this sequence multiplied by 2, and so on:

  1  2  2  2  4  3 ...

  2  4  4  4  8  ...

  3  6  6  6  ...

  4  8  8  ...

  5 10  ...

  6 ...

  ...

MATHEMATICA

Nest[Flatten@Table[#[[n - i]]*i, {n, Length[#] + 1}, {i, n - 1}] &, {1, 2}, 4]

CROSSREFS

Cf. A007501 (number of terms produced by the Mathematica code after n iterations).

Cf. A283682, A283683.

Sequence in context: A054709 A121806 A056944 * A222819 A194319 A208609

Adjacent sequences:  A283678 A283679 A283680 * A283682 A283683 A283684

KEYWORD

nonn,tabl,nice,look

AUTHOR

Ivan Neretin, Mar 14 2017

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified May 12 11:55 EDT 2021. Contains 343821 sequences. (Running on oeis4.)