Partition related number triangles
Set partitions by number of blocks
editAll partitions of a set with n elements into k blocks are enumerated by the number triangle called Stirling numbers of the second kind. When some partitions are left out or treated as equivalent the new set of partitions is enumerated by a new number triangle. The number triangles for noncrossing partitions are symmetric.
All | Noncrossing | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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All | Triangle A008277 (Stirling2) with row sums A000110 (Bell)
Diagonal: A129506 |
Triangle A001263 (Narayana) with row sums A000108 (Catalan)
Diagonal: A000891 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Up to rot |
Triangle A152175 with row sums A084423
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Triangle A209805 with row sums A054357
Diagonal is probably A001246 (square Catalan numbers) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Up to rot & ref |
Triangle A152176 with row sums A084708
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Triangle A209612 with row sums A111275
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In the two illustrations above some set partitions, sharing the same colors, are grouped together.
They always correspond to the same integer partition, shown in the following illustration:
Triangle A008284 with row sums A000041
Diagonal and second knight moves diagonal: A000041 from 0th entry |
Lah numbers
editCounting not only the partitions (which gives the Stirling2 numbers shown above), but also the possible permutations within the blocks gives the triangle of unsigned Lah numbers A105278 (with the row sums A000262).
Also here one could define a noncrossing version (where T(4;2) would be 32 instead of 36).
Set partitions by type and by number of singletons
edit
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All
edit
All, All (by number of singletons)
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All, Noncrossing (by number of singletons)
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By type | ||||||
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All, All (by type) Triangle A211350
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Up to rotation
edit
Up to rot, All (by number of singletons)
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Up to rot, Noncrossing (by number of singletons)
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By type | ||||||
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Up to rot, All (by type) Triangle A211352
The diagonal is A003239. According to the OEIS page this is also the number of necklaces with n white and n black beads. |
Up to rotation and reflection
edit
Up to rot & ref, All (by number of singletons)
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Up to rot & ref, Noncrossing (by number of singletons)
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By type | ||||||
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Up to rot & ref, All (by type) Triangle A211354
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Subsequences
editThe by type triangles have some interesting subsequences which are highlighted by colors in the files linked below.
Color | Size | Integer partition |
---|---|---|
■ | 2n | 1 * n + n * 1 |
■ | 2n | n * 2 |
□ | 2n | 2 * n |
■ | 2n + 1 | 1 * n + (n+1) * 1 |
■ | 2n + 1 | n * 1 + 1 * (n+1) |
□ | 2n + 1 | 1 * n + 1 * (n+1) |
1 * n + n * 1 stands for the integer partition with one addend n and n addends 1, etc.
All | Noncrossing | |||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
All |
Triangle A211350
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Triangle A211351
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Up to rot |
Triangle A211352
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Triangle A211353
| ||||||||||||||||
Up to rot & ref |
Triangle A211354
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Triangle A211355
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N = 0, 1, 2, 3, 4, 5, 6... A000984(N) = 1, 2, 6, 20, 70, 252, 924... (central binomial coefficients)
- The sequence 1,6,20,70,252,924... appears as All, All ■ and All, Noncrossing ■ :
- There is one partition of a 2-set that has a 1-block (i.e. a singleton) and another singleton.
- For n>1 there are A000984(n) partitions of a 2n-set that have an n-block and n singletons.
N = 0, 1, 2, 3, 4, 5... A001700(N) = 1, 3, 10, 35, 126, 462...
- All, All □ :
- There are A001700(n+1) partitions of a 2n-set that have two n-blocks.
- All, All ■ and All, Noncrossing ■ :
- There is one partition of a 3-set that has a 1-block (i.e. a singleton) and two other singletons.
- For n>1 there are A001700(n) partitions of a (2n+1)-set that have an n-block and n+1 singletons.
- All, All ■ and All, Noncrossing ■ :
- There are A001700(n) partitions of a (2n+1)-set that have an (n+1)-block and n singletons.
- All, All □ :
- There are A001700(n) partitions of a (2n+1)-set that have an (n+1)-block and an n-block.
N+ = 1, 2, 3, 4, 5, 6... A003239(N+) = 1, 2, 4, 10, 26, 80...
- Up to rot, All ■ and Up to rot, Noncrossing ■ :
- There are A003239(n) necklaces of length 2n with n black and n white beads.
- Up to rot & ref, All ■ ■ □ and Up to rot & ref, Noncrossing ■ ■ :
- There are A003239(n) free necklaces (or bracelets) of length 2n+1 with n black and n+1 white beads.