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A080341
Sum of the first n terms that are congruent to 1, 4 or 5 mod 6 (A047259).
1
1, 5, 10, 17, 27, 38, 51, 67, 84, 103, 125, 148, 173, 201, 230, 261, 295, 330, 367, 407, 448, 491, 537, 584, 633, 685, 738, 793, 851, 910, 971, 1035, 1100, 1167, 1237, 1308, 1381, 1457, 1534, 1613, 1695, 1778, 1863, 1951, 2040, 2131, 2225, 2320, 2417
OFFSET
1,2
COMMENTS
Number of edges needed in a sector of a hexagon of size n paved by rhombi coming from triangular/hexagonal lattices.
FORMULA
a(n) = n^2+(n+1)/3 with integer division, that is n mod 3 = 0 : n^2+n/3 n mod 3 = 1 : n^2+(n-1)/3 n mod 3 = 2 : n^2+(n+1)/3.
G.f.: x*(1+3*x+x^2+x^3)/(1-x)^3/(1+x+x^2). [Colin Barker, Feb 12 2012]
MATHEMATICA
Accumulate[Select[Range[100], MemberQ[{1, 4, 5}, Mod[#, 6]]&]] (* Harvey P. Dale, Aug 16 2012 *)
PROG
(Java or beanShell script) for(int i=1, s=m=0; i<40; i++) { m= i%6; if((m==1)||(m==4)||(m==5)) System.out.print((s+=i)+", "); } for(int i=1; i<20; i++) System.out.print((i*i+(i+1)/3)+" ");
CROSSREFS
Cf. A047259.
Sequence in context: A076598 A374539 A306011 * A271328 A086653 A215449
KEYWORD
easy,nonn
AUTHOR
Christian Mercat (Integer.Sequence(AT)entrelacs.net), Mar 20 2003
STATUS
approved