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 A033638 Quarter-squares plus 1 (that is, a(n) = A002620(n) + 1). 45
 1, 1, 2, 3, 5, 7, 10, 13, 17, 21, 26, 31, 37, 43, 50, 57, 65, 73, 82, 91, 101, 111, 122, 133, 145, 157, 170, 183, 197, 211, 226, 241, 257, 273, 290, 307, 325, 343, 362, 381, 401, 421, 442, 463, 485, 507, 530, 553, 577, 601, 626, 651, 677, 703, 730, 757, 785, 813, 842 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Fill an infinity X infinity matrix with numbers so that 1..n^2 appear in the top left n X n corner for all n; write down the minimal elements in the rows and columns and sort into increasing order; maximize this list in the lexicographic order. a(n) = A001055(prime^n), number of factorizations. - Reinhard Zumkeller, Dec 29 2001 Locations of right angle turns in Ulam square spiral. - Donald S. McDonald, Jan 09 2003 a(n-1) (for n >= 1) is also the number u of unique Fibonacci/Lucas type sequences generated (the total number t of these sequences being a triangular number). Sum(n+1)=t. Then u=Sum((n+1/2) minus 0.5 for odd terms) except for the initial term. E.g., u=13: (n=6)+1 = 7; then 7/2 - 0.5 =3. So u = Sum(1, 1, 1, 2, 2, 3, 3) = 13. - Marco Matosic, Mar 11 2003 Number of (3412,123)-avoiding involutions in S_n. Schur's Theorem (1905): the maximum number of mutually commuting linearly independent complex matrices of order n is floor((n^2)/4) + 1. Jacobson gave a simpler proof 40 years later, generalizing from algebraically closed fields to arbitrary fields. 54 years after that, Mirzakhani gave an even simpler proof. - Jonathan Vos Post, Apr 03 2007 Let A be the Hessenberg n X n matrix defined by: A[1,j]=j mod 2, A[i,i]:=1, A[i,i-1]=-1, and A[i,j]=0 otherwise. Then, for n >= 1, a(n)=(-1)^(n-1)*coeff(charpoly(A,x),x). - Milan Janjic, Jan 24 2010 Except for the initial two terms, A033638 gives iterates of the nonsquare function: c(n) = f(c(n-1)), where f(n) = A000037(n) = n + floor(1/2 + sqrt(n)) = n-th nonsquare, starting with c(1)=2. - Clark Kimberling, Dec 28 2010 For n >= 1: for all permutations of [0..n-1]: number of distinct values taken by Sum_{k=0..n-1} (k mod 2) * pi(k). - Joerg Arndt, Apr 22 2011 First differences are A110654. - Jon Perry, Sep 12 2012 Number of (weakly) unimodal compositions of n with maximal part <= 2, see example. - Joerg Arndt, May 10 2013 Equals the sum of terms in upward sloping diagonals of an infinite lower triangle with 1's in the leftmost column and the natural numbers in all other columns. - Gary W. Adamson, Jan 29 2014 a(n) is the number of permutations of length n avoiding both 213 and 321 in the classical sense which are breadth-first search reading words of increasing unary-binary trees. For more details, see the entry for permutations avoiding 231 at A245898. - Manda Riehl, Aug 05 2014 Number of partitions of n with no more than 2 parts > 1, and either first part >= n-k+1 and second part = n-k+1 or first part = n-k and second part <= n-k for 1 <= k <= n. - Wouter Meeussen, Feb 22 2015 Number of possible values for the area of a polyomino whose perimeter is 2n + 4. - Luc Rousseau, May 10 2018 LINKS Reinhard Zumkeller, Table of n, a(n) for n = 0..10000 H. Cheballah, S. Giraudo, R. Maurice, Combinatorial Hopf algebra structure on packed square matrices, arXiv preprint arXiv:1306.6605 [math.CO], 2013. E. S. Egge, Restricted 3412-Avoiding Involutions: Continued Fractions, Chebyshev Polynomials and Enumerations, Thm. 6.6, arXiv:math/0307050 [math.CO], 2003. D. C. Fielder & C. O. Alford, An investigation of sequences derived from Hoggatt Sums and Hoggatt Triangles, Application of Fibonacci Numbers, 3 (1990) 77-88. Proceedings of 'The Third Annual Conference on Fibonacci Numbers and Their Applications,' Pisa, Italy, July 25-29, 1988. (Annotated scanned copy) Nathan Jacobson, Schur's theorems on commutative matrices, Bull. Amer. Math. Soc. 50 (1944) 431-436. M. Mirzakhani, A Simple Proof of a Theorem of Schur, The American Mathematical Monthly, Vol. 105, No. 3 (Mar 1998), pp. 260-262. D. Necas, I. Ohlidal, Consolidated series for efficient calculation of the reflection and transmission in rough multilayers, Optics Express, Vol. 22, 2014, No. 4; DOI:10.1364/OE.22.004499. See Table 1. I. Schur, Neue Begründung der Theorie der Gruppencharaktere, Sitzungsberichte der Königlich Preussischen Akademie der Wissenschaften zu Berlin (1905), 406-432. Index entries for linear recurrences with constant coefficients, signature (2,0,-2,1). FORMULA a(n) = ceiling((n^2+3)/4) = ( (7 + (-1)^n)/2 + n^2 )/4. Numbers of the form n^2 + 1 or n^2 + n + 1. - Donald S. McDonald, Jan 09 2003 G.f.: (1-x+x^3)/((1-x)^2*(1-x^2)); a(n) = a(n-1) + a(n-2) - a(n-3) + 1. - Jon Perry, Jul 07 2004 a(n) = a(n-2) + n - 1. - Paul Barry, Jul 14 2004 a(0) = 1; a(1) = 1; for n > 1 a(n) = a(n-1) + round(sqrt(a(n-1))). - Jonathan Vos Post, Jan 19 2006 a(n) = floor((n^2)/4) + 1. a(n) = 2*a(n-1) - 2*a(n-3) + a(n-4) for n > 3. - Philippe Deléham, Nov 03 2008 a(0) = a(1) = 1, a(n) = a(n-1) + ceiling(sqrt(a(n-2))) for n > 1. - Jonathan Vos Post, Oct 08 2011 a(n) = floor(b(n)) with b(n) = b(n-1) + n/(1+e^(1/n)) and b(0)= 1. - Richard R. Forberg, Jun 08 2013 Construct an infinite triangular matrix with 1's in the leftmost column and the natural numbers in all other columns but shifted down twice. Square the triangle and the sequence is the leftmost column vector. - Gary W. Adamson, Jan 27 2014 a(n) = a(n-1) + floor(n/2). - Michel Lagneau, Jul 11 2014 From Ilya Gutkovskiy, Oct 07 2016: (Start) E.g.f.: (exp(-x) + (7 + 2*x + 2*x^2)*exp(x))/8. a(n) = Sum_{k=0..n} A123108(k). Convolution of A008619 and A179184. (End) a(n) = (n^2 - n + 4)/2 - a(n-1) for n >= 1. - Kritsada Moomuang, Aug 03 2019 EXAMPLE First 4 rows can be taken to be 1,2,5,10,17,...; 3,4,6,11,18,...; 7,8,9,12,19,...; 13,14,15,16,20,... Ulam square spiral = 7 8 9 / 6 1 2 / 5 4 3 /...; changes of direction (right-angle) at 1 2 3 5 7 ... From Joerg Arndt, May 10 2013: (Start) The a(7)=13 unimodal compositions of 7 with maximal part <=2 are 01:  [ 1 1 1 1 1 1 1 ] 02:  [ 1 1 1 1 1 2 ] 03:  [ 1 1 1 1 2 1 ] 04:  [ 1 1 1 2 1 1 ] 05:  [ 1 1 1 2 2 ] 06:  [ 1 1 2 1 1 1 ] 07:  [ 1 1 2 2 1 ] 08:  [ 1 2 1 1 1 1 ] 09:  [ 1 2 2 1 1 ] 10:  [ 1 2 2 2 ] 11:  [ 2 1 1 1 1 1 ] 12:  [ 2 2 1 1 1 ] 13:  [ 2 2 2 1 ] (End) G.f. = 1 + x + 2*x^2 + 3*x^3 + 5*x^4 + 7*x^5 + 10*x^6 + 13*x^7 + 17*x^8 + ... MAPLE with(combstruct):ZL:=[st, {st=Prod(left, right), left=Set(U, card=r), right=Set(U, card=3)}, unlabeled]: subs(r=1, stack): seq(count(subs(r=2, ZL), size=m), m=6..62); # Zerinvary Lajos, Mar 09 2007 A033638 := proc(n)         1+floor(n^2/4) ; end proc: # R. J. Mathar, Jul 13 2012 MATHEMATICA a[n_] := a[n] = 2a[n - 1] - 2a[n - 3] + a[n - 4]; a = a = 1; a = 2; a = 3; Array[a, 54, 0] (* Robert G. Wilson v, Mar 28 2011 *) LinearRecurrence[{2, 0, -2, 1}, {1, 1, 2, 3}, 60] (* Robert G. Wilson v, Sep 16 2012 *) PROG (PARI) {a(n) = n^2\4 + 1} /* Michael Somos, Apr 03 2007 */ (Haskell) a033638 = (+ 1) . (`div` 4) . (^ 2)  -- Reinhard Zumkeller, Apr 06 2012 (MAGMA) [n^2 div 4 + 1: n in [0.. 50]]; // Vincenzo Librandi, Jul 31 2016 CROSSREFS Equals A002620 + 1. Cf. A002878, A004652, A002984, A083479. Cf. A002522 is the odd indexes of this sequence. Sequence in context: A075353 A132278 A025700 * A194205 A136413 A177337 Adjacent sequences:  A033635 A033636 A033637 * A033639 A033640 A033641 KEYWORD easy,nonn AUTHOR Tanya Y. Berger-Wolf (tanyabw(AT)uiuc.edu) STATUS approved

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Last modified October 13 23:40 EDT 2019. Contains 327983 sequences. (Running on oeis4.)