The OEIS is supported by the many generous donors to the OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A355145 Triangle read by rows: T(n,k) is the number of primitive subsets of {1,...,n} of cardinality k; n>=0, 0<=k<=ceiling(n/2). 2
 1, 1, 1, 1, 2, 1, 3, 1, 1, 4, 2, 1, 5, 5, 2, 1, 6, 7, 3, 1, 7, 12, 10, 3, 1, 8, 16, 15, 5, 1, 9, 22, 26, 13, 2, 1, 10, 28, 38, 22, 4, 1, 11, 37, 66, 60, 26, 4, 1, 12, 43, 80, 76, 35, 6, 1, 13, 54, 123, 156, 111, 41, 6, 1, 14, 64, 161, 227, 180, 74, 12 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,5 COMMENTS A set is primitive if it does not contain distinct i and j such that i divides j. For n >= 2, the alternating row sums equal -1. LINKS Marcel K. Goh and Jonah Saks, Alternating-sum statistics for certain sets of integers, arXiv:2206.12535 [math.CO], 2022. FORMULA Sum_{k=1..ceiling(n/2)} k * T(n,k) = A087077(n). - Alois P. Heinz, Jun 24 2022 EXAMPLE Triangle T(n,k) begins:    n/k 0  1  2  3  4  5  6  7  8  9 10 11 12     0  1     1  1  1     2  1  2     3  1  3  1     4  1  4  2     5  1  5  5  2     6  1  6  7  3     7  1  7 12 10  3     8  1  8 16 15  5     9  1  9 22 26 13  2    10  1 10 28 38 22  4    11  1 11 37 66 60 26  4    12  1 12 43 80 76 35  6    ... For n=6 and k=3 the T(6,3) = 3 primitive sets are {2,3,5}, {3,4,5}, and {4,5,6}. CROSSREFS Columns k=0..2 give: A000012, A000027, A161664. Row sums give A051026. T(2n,n) gives A174094. T(2n-1,n) gives A192298 for n>=1. Cf. A087077, A087086. Sequence in context: A174066 A089178 A187489 * A116599 A138121 A138151 Adjacent sequences:  A355141 A355142 A355144 * A355146 A355147 A355149 KEYWORD nonn,tabf AUTHOR Marcel K. Goh, Jun 20 2022 STATUS approved

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

Last modified August 14 17:12 EDT 2022. Contains 356122 sequences. (Running on oeis4.)