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!)
A212139 Triangular array:  T(n,k) is the number of k-element subsets of {1,...,n} that satisfy mean=median. 4
1, 2, 1, 3, 3, 1, 4, 6, 2, 1, 5, 10, 4, 3, 1, 6, 15, 6, 7, 2, 1, 7, 21, 9, 13, 5, 3, 1, 8, 28, 12, 22, 10, 8, 2, 1, 9, 36, 16, 34, 18, 18, 6, 3, 1, 10, 45, 20, 50, 30, 36, 14, 9, 2, 1, 11, 55, 25, 70, 48, 66, 32, 23, 7, 3, 1, 12, 66, 30, 95, 72, 114, 64, 55, 20, 10, 2, 1 (list; table; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Row sums:  A212146.

LINKS

Table of n, a(n) for n=1..78.

EXAMPLE

First 7 rows:

1

2...1

3...3....1

4...6....2...1

5...10...4...3....1

6...15...6...7....2...1

7...21...9...13...5...3...1

T(5,3) counts these subsets: {1,2,3}, 1,3,5}, {2,3,4}, {3,4,5}.

MATHEMATICA

t[n_, k_] := t[n, k] = Count[Map[Median[#] == Mean[#] &, Subsets[Range[n], {k}]], True]

Flatten[Table[t[n, k], {n, 1, 12}, {k, 1, n}]]

TableForm[Table[t[n, k], {n, 1, 12}, {k, 1, n}]]

s[n_] := Sum[t[n, k], {k, 1, n}]

Table[s[n], {n, 1, 22}]   (* A212146 *)

(% - 1)/2  (* A212147 *)

(* Peter J. C. Moses, May 01 2012 *)

CROSSREFS

Cf. A212138.

Sequence in context: A073020 A090349 A157379 * A093430 A074659 A131251

Adjacent sequences:  A212136 A212137 A212138 * A212140 A212141 A212142

KEYWORD

nonn,tabl

AUTHOR

Clark Kimberling, May 06 2012

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 April 6 10:05 EDT 2020. Contains 333273 sequences. (Running on oeis4.)