A004006 -id:A004006 - cp's OEIS frontend

A055795 a(n) = binomial(n,4) + binomial(n,2).

0, 1, 3, 7, 15, 30, 56, 98, 162, 255, 385, 561, 793, 1092, 1470, 1940, 2516, 3213, 4047, 5035, 6195, 7546, 9108, 10902, 12950, 15275, 17901, 20853, 24157, 27840, 31930, 36456, 41448, 46937, 52955, 59535, 66711, 74518, 82992, 92170, 102090, 112791, 124313, 136697

Offset: 1

Clark Kimberling, May 28 2000

Answer to the question: if you have a tall building and 4 plates and you need to find the highest story from which a plate thrown does not break, what is the number of stories you can handle given n tries?
If Y is a 2-subset of an n-set X then, for n >= 4, a(n-3) is the number of 4-subsets of X which do not have exactly one element in common with Y. - Milan Janjic, Dec 28 2007
Antidiagonal sums of A139600. - Johannes W. Meijer, Apr 29 2011
Also the number of maximal cliques in the n-tetrahedral graph for n > 5. - Eric W. Weisstein, Jun 12 2017
Mark each point on an 8^(n-2) grid with the number of points that are visible from the point; for n > 3, a(n) is the number of distinct values in the grid. - Torlach Rush, Mar 25 2021
Antidiagonal sums of both A057145 and also A134394 yield this sequence without the initial term 0. - Michael Somos, Nov 23 2021

James Spahlinger, Table of n, a(n) for n = 1..1000
Michael Boardman, The Egg-Drop Numbers, Mathematics Magazine, 77 (2004), 368-372.
Milan Janjic, Two Enumerative Functions
Alexsandar Petojevic, The Function vM_m(s; a; z) and Some Well-Known Sequences, Journal of Integer Sequences, Vol. 5 (2002), Article 02.1.7
Eric Weisstein's World of Mathematics, Johnson Graph
Eric Weisstein's World of Mathematics, Maximal Clique
Eric Weisstein's World of Mathematics, Tetrahedral Graph
Index entries for linear recurrences with constant coefficients, signature (5,-10,10,-5,1).

T(2n+1, n), array T as in A055794. Cf. A004006, A000127.

Cf. A134394, A051601, A057145.

[n*(n^3-6*n^2+23*n-18)/24: n in [1..100]]; // Wesley Ivan Hurt, Sep 29 2013

A055795:=n->binomial(n,4)+binomial(n,2); # Zerinvary Lajos, Jul 24 2006

Table[Binomial[n, 4] + Binomial[n, 2], {n, 50}] (* Vladimir Joseph Stephan Orlovsky, May 24 2009 *)
Table[n (n^3 - 6 n^2 + 23 n - 18)/24, {n, 100}] (* Wesley Ivan Hurt, Sep 29 2013 *)
LinearRecurrence[{5, -10, 10, -5, 1}, {0, 1, 3, 7, 15}, 50] (* Harvey P. Dale, Dec 07 2015 *)
Total[Binomial[Range[20], #] & /@ {2, 4}] (* Eric W. Weisstein, Dec 01 2017 *)
CoefficientList[Series[x (-1 + 2 x - 2 x^2)/(-1 + x)^5, {x, 0, 20}], x] (* Eric W. Weisstein, Dec 01 2017~ *)

A055795(n):=n*(n^3-6*n^2+23*n-18)/24$ makelist(A055795(n), n, 1, 100); /* Wesley Ivan Hurt, Sep 29 2013 */

a(n)= n*(n^3-6*n^2+23*n-18)/24 \\ Wesley Ivan Hurt, Sep 29 2013

a(n) = A000127(n)-1. Differences give A000127.
a(1) = 1; a(n) = a(n-1) + 1 + A004006(n-1).
a(n+1) = C(n, 1) + C(n, 2) + C(n, 3) + C(n, 4). - James Sellers, Mar 16 2002
Row sums of triangle A134394. Also, binomial transform of [1, 2, 2, 2, 1, 0, 0, 0, ...]. - Gary W. Adamson, Oct 23 2007
O.g.f.: -x^2(1-2x+2x^2)/(x-1)^5. a(n) = A000332(n) + A000217(n-1). - R. J. Mathar, Apr 13 2008
a(n) = n*(n^3 - 6*n^2 + 23*n - 18)/24. - Gary Detlefs, Dec 08 2011
a(n) = 5*a(n-1) - 10*a(n-2) + 10*a(n-3) - 5*a(n-4) + a(n-5); a(1)=0, a(2)=1, a(3)=3, a(4)=7, a(5)=15. - Harvey P. Dale, Dec 07 2015

Better description from Leonid Broukhis, Oct 24 2000
Edited by Zerinvary Lajos, Jul 24 2006
Offset corrected and Sellers formula adjusted by Gary Detlefs, Nov 28 2011

A062025 a(n) = n(13n^2 - 7)/6.

Original entry on oeis.org

0, 1, 15, 55, 134, 265, 461, 735, 1100, 1569, 2155, 2871, 3730, 4745, 5929, 7295, 8856, 10625, 12615, 14839, 17310, 20041, 23045, 26335, 29924, 33825, 38051, 42615, 47530, 52809, 58465, 64511, 70960, 77825, 85119, 92855, 101046, 109705, 118845, 128479, 138620, 149281

Offset: 0

N. J. A. Sloane, Aug 02 2001

Harry J. Smith, Table of n, a(n) for n = 0..1000
T. P. Martin, Shells of atoms, Phys. Reports, 273 (1996), 199-241, eq. (11).
Index entries for linear recurrences with constant coefficients, signature (4, -6, 4, -1).

1/12*t*(n^3-n)+n for t = 2, 4, 6, ... gives A004006, A006527, A006003, A005900, A004068, A000578, A004126, A000447, A004188, A004466, A004467, A007588, A062025, A063521, A063522, A063523.

Table[n(13n^2-7)/6,{n,0,80}] (* Vladimir Joseph Stephan Orlovsky, Apr 18 2011 *)

a(n) = n*(13*n^2 - 7)/6 \\ Harry J. Smith, Jul 29 2009

From G. C. Greubel, Sep 01 2017: (Start)
a(n) = 4*a(n-1) - 6*a(n-2) + 4*a(n-3) - a(n-4).
G.f.: (x + 11*x^2 + x^3)/(1 - x)^4.
E.g.f.: (x/6)*(6 + 39*x + 13*x^2)*exp(x). (End)

A063523 a(n) = n(8n^2 - 5)/3.

Original entry on oeis.org

0, 1, 18, 67, 164, 325, 566, 903, 1352, 1929, 2650, 3531, 4588, 5837, 7294, 8975, 10896, 13073, 15522, 18259, 21300, 24661, 28358, 32407, 36824, 41625, 46826, 52443, 58492, 64989, 71950, 79391, 87328, 95777, 104754, 114275, 124356, 135013, 146262, 158119, 170600

Offset: 0

N. J. A. Sloane, Aug 02 2001

Also as a(n)=(1/6)*(16*n^3-10*n), n>0: structured octagonal anti-diamond numbers (vertex structure 17) (Cf. A100187 = alternate vertex; A100188 = structured anti-diamonds; A100145 for more on structured numbers). - James A. Record (james.record(AT)gmail.com), Nov 07 2004

Harry J. Smith, Table of n, a(n) for n = 0..1000
T. P. Martin, Shells of atoms, Phys. Reports, 273 (1996), 199-241, eq. (11).
Index entries for linear recurrences with constant coefficients, signature (4, -6, 4, -1).

1/12*t*(n^3-n)+n for t = 2, 4, 6, ... gives A004006, A006527, A006003, A005900, A004068, A000578, A004126, A000447, A004188, A004466, A004467, A007588, A062025, A063521, A063522, A063523.

Table[n(8n^2-5)/3,{n,0,80}] (* Vladimir Joseph Stephan Orlovsky, Apr 18 2011 *)
LinearRecurrence[{4,-6,4,-1},{0,1,18,67},81] (* or *) CoefficientList[ Series[ (x+14 x^2+x^3)/(x-1)^4,{x,0,80}],x] (* Harvey P. Dale, Jul 11 2011 *)

a(n) = n*(8*n^2 - 5)/3 \\ Harry J. Smith, Aug 25 2009

a(0)=0, a(1)=1, a(2)=18, a(3)=67, a(n)=4*a(n-1)-6*a(n-2)+4*a(n-3)- a(n-4). - Harvey P. Dale, Jul 11 2011
G.f.: (x+14*x^2+x^3)/(x-1)^4. - Harvey P. Dale, Jul 11 2011
E.g.f.: (x/3)*(3 + 24*x + 8*x^2)*exp(x). - G. C. Greubel, Sep 01 2017

A033547 Otto Haxel's guess for magic numbers of nuclear shells.

Original entry on oeis.org

0, 2, 6, 14, 28, 50, 82, 126, 184, 258, 350, 462, 596, 754, 938, 1150, 1392, 1666, 1974, 2318, 2700, 3122, 3586, 4094, 4648, 5250, 5902, 6606, 7364, 8178, 9050, 9982, 10976, 12034, 13158, 14350, 15612, 16946, 18354, 19838, 21400, 23042, 24766, 26574, 28468

Offset: 0

Wolfdieter Lang

O. Haxel gave a construction procedure. The formulas are due to Wolfdieter Lang.

G. C. Greubel, Table of n, a(n) for n = 0..1000
O. Haxel, Die Entstehung des Schalenmodells der Atomkerne, Physikalische Blätter, vol. 50, p. 339, 1994.
O. Haxel et al., On the "Magic Numbers" in Nuclear Structure, Phys. Rev., 75 (1949), 1766.
V. Ladma, Magic Numbers
Index entries for linear recurrences with constant coefficients, signature (4,-6,4,-1).

Equals 2*A004006, partial sums of A014206, 2*(partial sums of A000124).

Cf. A018226, A046127.

List([0..50], n-> n*(n^2+5)/3); # G. C. Greubel, Oct 12 2019

[n*(n^2+5)/3 : n in [0..50]]; // Wesley Ivan Hurt, Apr 05 2015

A033547:=n->n*(n^2+5)/3: seq(A033547(n), n=0..50); # Wesley Ivan Hurt, Apr 05 2015

Table[n(n^2+5)/3, {n,0,50}] (* Harvey P. Dale, Apr 07 2011 *)
LinearRecurrence[{4, -6, 4, -1}, {0, 2, 6, 14}, 50] (* Vincenzo Librandi, Apr 06 2015 *)

a(n)=n*(n^2+5)/3 \\ Charles R Greathouse IV, Jun 25 2017

[n*(n^2+5)/3 for n in range(50)] # G. C. Greubel, Oct 12 2019

a(n) = n*(n^2 + 5)/3.
G.f.: 2*x*(1 - x + x^2)/(1-x)^4.
a(n) = 4*a(n-1) - 6*a(n-2) + 4*a(n-3) - a(n-4). - Wesley Ivan Hurt, Apr 05 2015
E.g.f.: x*(6 + 3*x + x^2)*exp(x)/3. - G. C. Greubel, Oct 12 2019
a(n) = A046127(n+1) - 2. - Jianing Song, Feb 03 2024

A266428 T(n,k)=Number of nXk binary arrays with rows and columns lexicographically nondecreasing and column sums nondecreasing.

Original entry on oeis.org

2, 3, 3, 4, 7, 4, 5, 14, 13, 5, 6, 25, 39, 22, 6, 7, 41, 106, 96, 34, 7, 8, 63, 259, 404, 212, 50, 8, 9, 92, 574, 1556, 1391, 433, 70, 9, 10, 129, 1170, 5365, 8764, 4383, 826, 95, 10, 11, 175, 2223, 16585, 49894, 45907, 12758, 1493, 125, 11, 12, 231, 3982, 46463, 251381

Offset: 1

R. H. Hardin, Dec 29 2015

Table starts
..2...3....4......5........6..........7............8............9
..3...7...14.....25.......41.........63...........92..........129
..4..13...39....106......259........574.........1170.........2223
..5..22...96....404.....1556.......5365........16585........46463
..6..34..212...1391.....8764......49894.......251381......1122721
..7..50..433...4383....45907.....448649......3889553.....29520031
..8..70..826..12758...223075....3825307.....59155748....798834778
..9..95.1493..34611..1005991...30555624....861030491..21325003746
.10.125.2575..88206..4224203..227542455..11809616668.546283341439
.11.161.4270.212609.16588684.1579153474.151566391972

			Some solutions for n=4 k=4
..0..0..0..0....0..0..1..1....0..0..0..1....0..0..1..1....0..0..0..1
..0..0..0..1....0..0..1..1....0..0..1..0....0..1..0..1....0..1..1..1
..0..0..1..1....1..1..0..1....0..1..1..1....0..1..1..1....0..1..1..1
..0..1..0..1....1..1..1..0....1..1..0..0....1..1..1..0....1..0..0..1

R. H. Hardin, Table of n, a(n) for n = 1..145

Column 1 and row 1 are A000027(n+1).
Column 2 is A002623.
Row 2 is A004006(n+1).

Empirical for column k:
k=1: a(n) = 2*a(n-1) -a(n-2)
k=2: a(n) = 3*a(n-1) -2*a(n-2) -2*a(n-3) +3*a(n-4) -a(n-5)
k=3: [order 12] Empirical for row n:
n=1: a(n) = n + 1
n=2: a(n) = (1/6)*n^3 + (1/2)*n^2 + (4/3)*n + 1
n=3: [polynomial of degree 6]
n=4: [polynomial of degree 11]
n=5: [polynomial of degree 19]
n=6: [polynomial of degree 33]
n=7: [polynomial of degree 57]

A177342 a(n) = (4n^3-3n^2+5*n-3)/3.

Original entry on oeis.org

1, 9, 31, 75, 149, 261, 419, 631, 905, 1249, 1671, 2179, 2781, 3485, 4299, 5231, 6289, 7481, 8815, 10299, 11941, 13749, 15731, 17895, 20249, 22801, 25559, 28531, 31725, 35149, 38811, 42719, 46881, 51305, 55999, 60971, 66229, 71781, 77635

Offset: 1

Bruno Berselli, May 06 2010 - Nov 27 2010

This sequence is related to the fourth powers (A000583) by n^4 = n*a(n) - Sum_{i=1..n-1} a(i) - (n-1), with n>1.

Also, n*a(n) - Sum_{i=1..n-1} a(i) provides the first column of A162624 and the second column of A162622 (or A162623). - Bruno Berselli, revised Dec 14 2012

B. Berselli, Table of n, a(n) for n = 1..10000
Index entries for linear recurrences with constant coefficients, signature (4,-6,4,-1).

First differences: 2*A084849.
Partial sums: A178073.
Cf. A174723, A162622-A162624.

[(4*n^3-3*n^2+5*n-3)/3: n in [1..39]]; // Bruno Berselli, Aug 24 2011

I:=[1,9,31,75]; [n le 4 select I[n] else 4*Self(n-1)-6*Self(n-2)+4*Self(n-3)-Self(n-4): n in [1..50]]; // Vincenzo Librandi, Aug 19 2013

CoefficientList[Series[(1 + 5 x + x^2 + x^3) / (1 - x)^4, {x, 0, 50}], x] (* Vincenzo Librandi, Aug 19 2013 *)
Table[(4 n^3 - 3 n^2 + 5 n - 3)/3, {n, 1, 40}] (* Bruno Berselli, Feb 17 2015 *)
LinearRecurrence[{4,-6,4,-1},{1,9,31,75},40] (* Harvey P. Dale, Jul 31 2021 *)

a(n)=(4*n^3-3*n^2+5*n-3)/3 \\ Charles R Greathouse IV, Jun 23 2011

G.f.: x*(1 + 5*x + x^2 + x^3)/(1 - x)^4.
a(n) = 4*a(n-1) - 6*a(n-2) + 4*a(n-3) - a(n-4).
a(n) - a(-n) = 2*A004006(2n).
a(n) + a(-n) = -A002522(n).
a(n) = 1 + (n-1)*(4*n^2+n+6)/3 = 2*A174723(n)-1.

Formulae added and revised by Bruno Berselli, Feb 17 2015

A180985 Array T(n,k) = number of n X k binary matrices with rows and columns in lexicographically nondecreasing order.

Original entry on oeis.org

2, 3, 3, 4, 7, 4, 5, 14, 14, 5, 6, 25, 45, 25, 6, 7, 41, 130, 130, 41, 7, 8, 63, 336, 650, 336, 63, 8, 9, 92, 785, 2942, 2942, 785, 92, 9, 10, 129, 1682, 11819, 24520, 11819, 1682, 129, 10, 11, 175, 3351, 42305, 183010, 183010, 42305, 3351, 175, 11, 12, 231, 6280, 136564

Offset: 1

R. H. Hardin, Sep 30 2010

Differs from "number of inequivalent {0,1}-matrices of size n X k, modulo permutations of rows and columns", A241956, starting at T(2, 3) = 14 while A241956(2, 3) = 13. - M. F. Hasler, Apr 27 2022

			Table starts:
..2...3.....4.......5.........6...........7.............8................9
..3...7....14......25........41..........63............92..............129
..4..14....45.....130.......336.........785..........1682.............3351
..5..25...130.....650......2942.......11819.........42305...........136564
..6..41...336....2942.....24520......183010.......1202234..........6979061
..7..63...785...11819....183010.....2625117......33345183........371484319
..8..92..1682...42305...1202234....33345183.....836488618......18470742266
..9.129..3351..136564...6979061...371484319...18470742266.....818230288201
.10.175..6280..402910..36211867..3651371519..358194085968...31887670171373
.11.231.11176.1099694.170079565.32017940222.6148026957098.1096628939510047
.
All solutions for 3 X 3:
..0..0..0....0..0..0....0..0..0....0..0..0....0..0..0....0..0..0....0..0..0
..0..0..0....0..0..0....0..0..1....0..0..1....0..0..1....0..1..1....0..0..0
..0..0..1....0..1..1....0..1..0....0..0..1....0..1..1....0..1..1....1..1..1
.
..0..0..0....0..0..0....0..0..0....0..0..0....0..0..0....0..0..1....0..0..1
..0..0..1....0..1..1....0..0..1....0..1..1....0..1..1....0..1..0....0..1..0
..1..1..0....1..0..0....1..1..1....1..0..1....1..1..1....0..1..0....0..1..1
.
..0..0..1....0..0..1....0..0..1....0..0..1....0..0..1....0..0..1....0..0..1
..0..0..1....0..0..1....0..0..1....0..1..1....0..1..0....0..1..0....0..1..0
..0..1..0....0..0..1....0..1..1....0..1..1....1..0..0....1..1..0....1..0..1
.
..0..0..1....0..0..1....0..0..1....0..0..1....0..0..1....0..0..1....0..0..1
..0..1..0....0..0..1....0..1..1....0..1..1....0..0..1....0..1..1....0..1..1
..1..1..1....1..1..0....1..0..0....1..1..0....1..1..1....1..0..1....1..1..1
.
..0..0..0....0..0..1....0..0..1....0..0..1....0..1..1....0..1..1....0..1..1
..1..1..1....1..1..0....1..1..0....1..1..1....0..1..1....0..1..1....0..1..1
..1..1..1....1..1..0....1..1..1....1..1..1....0..1..1....1..0..0....1..0..1
...
..0..1..1....0..1..1....0..1..1....0..1..1....0..1..1....0..1..1....0..1..1
..0..1..1....1..0..0....1..0..0....1..0..0....1..0..1....1..0..1....1..0..1
..1..1..1....1..0..0....1..0..1....1..1..1....1..1..0....1..0..1....1..1..1
.
..0..1..1....1..1..1
..1..1..1....1..1..1
..1..1..1....1..1..1

Cf. A089006 (diagonal).
Cf. A004006 (row & column 2), A184138 (row & column 3).
Cf. A241956 (similar but different).

A180985(h,w,cnt=0)={ local(A=matrix(h,w), z(r,c)=!while(r1 && z(r,c), c--); while(c>1, A[r,c--]=0); while(r>1, A[r--,]=A[r+1,]); next(3))); break); cnt} \\ M. F. Hasler, Apr 27 2022

T(n,k) = T(k,n). T(1,k) = k+1. T(2,k) = A004006(k+1). T(3,k) = A184138(k). - M. F. Hasler, Apr 27 2022

A212013 Triangle read by rows: total number of pairs of states of the first n subshells of the nuclear shell model in which the subshells are ordered by energy level in increasing order.

Original entry on oeis.org

1, 3, 4, 7, 9, 10, 14, 17, 19, 20, 25, 29, 32, 34, 35, 41, 46, 50, 53, 55, 56, 63, 69, 74, 78, 81, 83, 84, 92, 99, 105, 110, 114, 117, 119, 120, 129, 137, 144, 150, 155, 159, 162, 164, 165, 175, 184, 192, 199, 205, 210, 214, 217, 219, 220, 231, 241, 250, 258, 265, 271, 276, 280, 283, 285, 286

Offset: 1

Omar E. Pol, Jul 15 2012

			Example 1: written as a triangle in which row i is related to the (i-1)st level of nucleus. Triangle begins:
    1;
    3,   4;
    7,   9,  10;
   14,  17,  19,  20;
   25,  29,  32,  34,  35;
   41,  46,  50,  53,  55,  56;
   63,  69,  74,  78,  81,  83,  84;
   92,  99, 105, 110, 114, 117, 119, 120;
  129, 137, 144, 150, 155, 159, 162, 164, 165;
  175, 184, 192, 199, 205, 210, 214, 217, 219, 220;
  ...
Column 1 gives positive terms of A004006. Right border gives positive terms of A000292. Row sums give positive terms of A006325.
Example 2: written as an irregular triangle in which row j is related to the j-th shell of nucleus. Note that in this case row 4 has only one term. Triangle begins:
    1;
    3,   4;
    7,   9,  10;
   14;
   17,  19,  20,  25;
   29,  32,  34,  35,  41;
   46,  50,  53,  55,  56,  63;
   69,  74,  78,  81,  83,  84,  92;
   99, 105, 110, 114, 117, 119, 120, 129;
  137, 144, 150, 155, 159, 162, 164, 165, 175;
  184, 192, 199, 205, 210, 214, 217, 219, 220, 231;
  ...

Paolo Xausa, Table of n, a(n) for n = 1..11325 (rows 1..150 of triangle, flattened).

Partial sums of A004736. Other versions are A210983, A212123, A213363, A213373.

Cf. A000292, A004006, A006325, A212012, A212014, A014370.

row =: monad define
    d=.>y
    < |. (+/d)-d
)
;}. row"0 <\ +/\ 1+i.11 NB. Vanessa McHale (vamchale(AT)gmail.com), Mar 01 2025

Accumulate[Flatten[Range[Range[15], 1, -1]]] (* Paolo Xausa, Mar 15 2025 *)

row(n) = vector(n, k, n*(n+1)*(n+2)/6 - (n-k)*(n-k+1)/2); \\ Michel Marcus, Mar 10 2025

a(n) = A212014(n)/2.
Let R = floor(sqrt(8*n+1)) and S = floor(R/2) + R mod 2; then a(n) = binomial(S,3) + n + (n-binomial(S,2))*(S*(S+3)-2*n-2)/4. - Gerald Hillier, Jan 16 2018
T(n,k) = n*(n+1)*(n+2)/6 - (n-k)*(n-k+1)/2. - Davide Rotondo, Mar 10 2025
G.f.: x*y*(1 - x + x^2*(1 - 3*y) - x^5*y^3 + x^3*y*(1 + y) - x^4*y*(1 - 2*y))/((1 - x)^4*(1 - x*y)^4). - Stefano Spezia, Mar 10 2025

More terms from Michel Marcus, Mar 10 2025

A212123 Total number of pairs of states of the first n subshells of the nuclear shell model in which the subshells are ordered by energy level in increasing order.

Original entry on oeis.org

1, 3, 4, 7, 8, 10, 14, 16, 19, 20, 25, 29, 32, 34, 35, 41, 46, 50, 53, 55, 56, 63, 68, 71, 77, 81, 82, 84, 92

Offset: 1

Omar E. Pol, Jun 03 2012

First differs from A213363 at a(12).

			Example 1: written as a triangle in which row i is related to the (i-1)st level of nucleus. Triangle begins:
1;
3,    4;
7,    8,  10;
14,  16,  19,  20;
25,  29,  32,  34,  35;
41,  46,  50,  53,  55,  56;
63,  68,  71,  77,  81,  82,  84;
...
Example 2: written as an irregular triangle in which row j is related to the j-th shell of nucleus. Note that in this case row 4 has only one term. Triangle begins:
1;
3,   4;
7,   8, 10;
14,
16, 19, 20, 25;
29, 32, 34, 35, 41;
46, 50, 53, 55, 56, 63;
68, 71, 77, 81, 82, 84, 92;
...

Partial sums of A212121. Other versions are A210983, A212013, A213363, A213373.

Cf. A000292, A004006, A212122, A212124.

a(n) = A212124(n)/2.

A027927 Number of plane regions after drawing (in general position) a convex n-gon and all its diagonals.

Original entry on oeis.org

1, 2, 5, 12, 26, 51, 92, 155, 247, 376, 551, 782, 1080, 1457, 1926, 2501, 3197, 4030, 5017, 6176, 7526, 9087, 10880, 12927, 15251, 17876, 20827, 24130, 27812, 31901, 36426, 41417, 46905, 52922, 59501, 66676, 74482, 82955, 92132, 102051, 112751, 124272, 136655, 149942, 164176, 179401

Offset: 2

Clark Kimberling

For n>=1, a(n+1) is the number of Grassmannian permutations that avoid a pattern, sigma, where sigma is a pattern of size 5 with exactly one descent. - Jessica A. Tomasko, Nov 15 2022

			a(2)=1 (segment traced twice has only exterior).

Vincenzo Librandi, Table of n, a(n) for n = 2..10000
Lapo Cioni and Luca Ferrari, Enumerative Results on the Schröder Pattern Poset, In: Dennunzio A., Formenti E., Manzoni L., Porreca A. (eds) Cellular Automata and Discrete Complex Systems, AUTOMATA 2017, Lecture Notes in Computer Science, vol 10248.
Michael Dairyko, Samantha Tyner, Lara Pudwell, and Casey Wynn, Non-contiguous pattern avoidance in binary trees, Electron. J. Combin. 19 (2012), no. 3, Paper 22, 21 pp. MR2967227. - From _N. J. A. Sloane_, Feb 01 2013
J. B. Gil and J. Tomasko, Restricted Grassmannian permutations, ECA 2:4 (2022) Article S4PP6.
Milan Janjić, Hessenberg Matrices and Integer Sequences, J. Int. Seq. 13 (2010) # 10.7.8.
Index entries for linear recurrences with constant coefficients, signature (5,-10,10,-5,1).

Cf. A006522 (does not count exterior of n-gon).

Cf. A000045, A000217, A004006, A027926, A228074.

List([2..50], n-> (n^4 -6*n^3 +23*n^2 -42*n +48)/24); # G. C. Greubel, Sep 06 2019

[(n^4 -6*n^3 +23*n^2 -42*n +48)/24: n in [2..50]]; // G. C. Greubel, Sep 06 2019

seq((n^4 -6*n^3 +23*n^2 -42*n +48)/24, n=2..50); # G. C. Greubel, Sep 06 2019

LinearRecurrence[{5,-10,10,-5,1 }, {1,2,5,12,26}, 50] (* Vincenzo Librandi, Feb 01 2012 *)
S[n_] :=n*(n+1)/2; Table[S[S[n]+2]/3, {n, 0, 50}] (* Waldemar Puszkarz, Jan 22 2016 *)

a(n)=n*(n^3-6*n^2+23*n-42)/24+2 \\ Charles R Greathouse IV, Jan 31 2012

[(n^4 -6*n^3 +23*n^2 -42*n +48)/24 for n in (2..50)] # G. C. Greubel, Sep 06 2019

a(n) = T(n, 2*n-4), T given by A027926.
a(n) = 1 + binomial(n, 4) + binomial(n-1, 2) = (n^4 - 6*n^3 + 23*n^2 - 42*n + 48)/24.
G.f.: x^2*(1 -3*x +5*x^2 -3*x^3 +x^4)/(1-x)^5. - Colin Barker, Jan 31 2012
a(n) = (1/6)*A152950(n-1)*A152948(n). - Bruno Berselli, Jan 31 2012
a(n) = A000217(A000217(n-2)+2)/3, a(n+1) - a(n) = A004006(n-1) for n > 2. - Waldemar Puszkarz, Jan 22 2016 [Adjusted for offset by Peter Munn, Jan 10 2023]
a(n) = 1 + Sum {i=3..5} binomial(n-1, i-1). - Jessica A. Tomasko, Nov 15 2022

New name from Len Smiley, Oct 19 2001

cp's OEIS Frontend

A055795 a(n) = binomial(n,4) + binomial(n,2).

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A062025 a(n) = n*(13*n^2 - 7)/6.

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A063523 a(n) = n*(8*n^2 - 5)/3.

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A033547 Otto Haxel's guess for magic numbers of nuclear shells.

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A266428 T(n,k)=Number of nXk binary arrays with rows and columns lexicographically nondecreasing and column sums nondecreasing.

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A177342 a(n) = (4*n^3-3*n^2+5*n-3)/3.

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A180985 Array T(n,k) = number of n X k binary matrices with rows and columns in lexicographically nondecreasing order.

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A062025 a(n) = n(13n^2 - 7)/6.

A063523 a(n) = n(8n^2 - 5)/3.

A177342 a(n) = (4n^3-3n^2+5*n-3)/3.