Opened 5 years ago

Last modified 5 years ago

## #17161 new enhancement

# Implement the unsigned Lah numbers

Reported by: | pluschny | Owned by: | |
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Priority: | minor | Milestone: | sage-6.4 |

Component: | combinatorics | Keywords: | Lah numbers |

Cc: | Merged in: | ||

Authors: | Reviewers: | ||

Report Upstream: | N/A | Work issues: | |

Branch: | Commit: | ||

Dependencies: | Stopgaps: |

### Description

Implement the unsigned Lah numbers which for n>=1, k>=1 are the number of ways a set of n elements can be partitioned into k nonempty linearly ordered subsets.

Lah numbers are defined for all integers n, k by the following formula if the Stirling numbers are extended to negative integers n, k in the way it is recommended by Graham/Knuth/Patashnik? in 'Concrete Mathematics' Section 6.1 (see Table 253). (This extension is implemented in GAP’s Stirling functions.)

def Lah(n,k): return sum(stirling_number1(n,j)*stirling_number2(j,k,"gap") for j in (k..n))

(Note how inconsistently at present this formula has to be written to get consistent results (because stirling_number1 and stirling_number2 are based on different, non-compatible algorithms! See also ticket #17159)

for n in (-6..6): print [Lah(n,k) for k in (-6..6)] 1, ., ., ., ., ., ., ., ., ., ., ., ., 30, 1, ., ., ., ., ., ., ., ., ., ., ., 300, 20, 1, ., ., ., ., ., ., ., ., ., ., 1200, 120, 12, 1, ., ., ., ., ., ., ., ., ., 1800, 240, 36, 6, 1, ., ., ., ., ., ., ., ., 720, 120, 24, 6, 2, 1, ., ., ., ., ., ., ., ., ., ., ., ., ., 1, ., ., ., ., ., ., ., ., ., ., ., ., ., 1, ., ., ., ., ., ., ., ., ., ., ., ., 2, 1, ., ., ., ., ., ., ., ., ., ., ., 6, 6, 1, ., ., ., ., ., ., ., ., ., ., 24, 36, 12, 1, ., ., ., ., ., ., ., ., ., 120, 240, 120, 20, 1, ., ., ., ., ., ., ., ., 720, 1800, 1200, 300, 30, 1,

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A possible implementation could be based on: