OFFSET
1,2
COMMENTS
Since the diagram of the symmetric representation of sigma is also the top view of the stepped pyramid described in A245092, and the diagram is also the top view of the staircase described in A244580, so we have that a(n) is also the height difference (or length of the vertical line segment) at the point (n,n) in the main diagonal of the mentioned structures.
a(n) is the number of occurrences of n in A240542. - Omar E. Pol, Dec 09 2016
EXAMPLE
Illustration of initial terms:
--------------------------------------------------------
Diagram with 15 Dyck paths
n A000203(n) a(n) to evaluate a(1)..a(10)
--------------------------------------------------------
. _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
1 1 1 |_| | | | | | | | | | | | | | |
2 3 2 |_ _|_| | | | | | | | | | | | |
3 4 2 |_ _| _|_| | | | | | | | | | |
4 7 0 |_ _ _| _|_| | | | | | | | |
5 6 2 |_ _ _| _| _ _|_| | | | | | |
6 12 1 |_ _ _ _| _| | _ _|_| | | | |
7 8 3 |_ _ _ _| |_ _|_| _ _|_| | |
8 15 0 |_ _ _ _ _| _| | _ _ _|_|
9 13 3 |_ _ _ _ _| | _|_| |
10 18 0 |_ _ _ _ _ _| _ _| _|
. |_ _ _ _ _ _| | _| _|
. |_ _ _ _ _ _ _| |_ _|
. |_ _ _ _ _ _ _| |
. |_ _ _ _ _ _ _ _|
. |_ _ _ _ _ _ _ _|
.
For n = 3 there are two Dyck paths that contain the point (3,3) so a(3) = 2.
For n = 4 there are no Dyck paths that contain the point (4,4) so a(4) = 0.
MATHEMATICA
a240542[n_] := Sum[(-1)^(k+1)Ceiling[(n+1)/k - (k+1)/2], {k, 1, Floor[(Sqrt[8n+1]-1)/2]}]
a259179[n_] := Module[{t=Table[0, n], k=1, d=1}, While[d<=n, t[[d]]+=1; d=a240542[++k]]; t] (* a(1..n) *)
a259179[102] (* Hartmut F. W. Hoft, Aug 06 2020 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Omar E. Pol, Aug 11 2015
EXTENSIONS
More terms from Omar E. Pol, Dec 09 2016
STATUS
approved