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!)
A330269 The number of semi-meanders with n top arches and concentric arches within the starting arch or a starting arch with length one. 1
1, 1, 2, 4, 8, 18, 42, 108, 282, 786, 2192, 6402, 18600, 55978, 167256, 514102, 1567976, 4896164, 15170630, 47957260, 150468678, 480371736, 1522649458, 4900568718, 15665593150, 50761432998, 163431901126, 532624603680, 1725349278270, 5650796083020, 18401781369182 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

LINKS

Jean-François Alcover, Table of n, a(n) for n = 1..46

FORMULA

a(1) = 1, for n >= 2, a(n) = Sum_{k=1..n-1} A000682(k).

EXAMPLE

For n = 5, a(5) = 8:

        /\                                  /\

       //\\                      /\        /  \          /\

      ///\\\      /\  /\        /  \      /  /\\    /\  //\\

   /\////\\\\, /\//\\//\\, /\/\//\/\\, /\//\//\\\, //\\///\\\,

                              /\

                 /\          //\\          starting arch

    /\  /\      //\\  /\    ///\\\        (1) (2) (3) (4)

   //\\//\\/\, ///\\\//\\, ////\\\\/\,     4 + 2 + 1 + 1 = 8.

MATHEMATICA

A000136 = Import["https://oeis.org/A000136/b000136.txt", "Table"][[All, 2]];

a[n_] := If[n == 1, 1, Sum[A000136[[k]]/k, {k, 1, n - 1}]];

a /@ Range[46] (* Jean-François Alcover, Feb 13 2020 *)

CROSSREFS

Cf. A000682.

Sequence in context: A261492 A027056 A024428 * A049075 A318797 A318850

Adjacent sequences:  A330266 A330267 A330268 * A330270 A330271 A330272

KEYWORD

nonn

AUTHOR

Roger Ford, Dec 07 2019

EXTENSIONS

More terms from Jinyuan Wang, Dec 08 2019

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 October 23 19:26 EDT 2021. Contains 348215 sequences. (Running on oeis4.)