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A186350
Adjusted joint rank sequence of (f(i)) and (g(j)) with f(i) before g(j) when f(i)=g(j), where f and g are the odd numbers and the triangular numbers. Complement of A186351.
20
1, 3, 5, 7, 8, 10, 11, 12, 14, 15, 16, 18, 19, 20, 22, 23, 24, 25, 27, 28, 29, 30, 31, 33, 34, 35, 36, 37, 39, 40, 41, 42, 43, 45, 46, 47, 48, 49, 50, 52, 53, 54, 55, 56, 57, 58, 60, 61, 62, 63, 64, 65, 66, 68, 69, 70, 71, 72, 73, 74, 76, 77, 78, 79, 80, 81, 82, 83, 85, 86, 87, 88, 89, 90, 91, 92, 93, 95, 96, 97, 98, 99, 100, 101, 102, 103, 105, 106, 107, 108, 109, 110, 111, 112, 113, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 138, 139, 140, 141
OFFSET
1,2
COMMENTS
Suppose that f and g are strictly increasing functions for which (f(i)) and (g(j)) are integer sequences. If 0<|d|<1, the sets F={f(i): i>=1} and G={g(j)+d: j>=1} are clearly disjoint. Let f^=(inverse of f) and g^=(inverse of g). When the numbers in F and G are jointly ranked, the rank of f(n) is a(n):=n+floor(g^(f(n))-d), and the rank of g(n)+d is b(n):=n+floor(f^(g(n))+d). Therefore, the sequences a and b are a complementary pair.
Although the sequences (f(i)) and (g(j)) may not be disjoint, the sequences (f(i)) and (g(j)+d) are disjoint, and this observation enables two types of adjusted joint rankings:
(1) if 0<d<1, we call a and b the "adjusted joint rank sequences of (f(i)) and (g(j)) with f(i) before g(j) when f(i)=g(j)"; (2) if -1<d<0, we call a and b the "adjusted joint rank sequences of (f(i)) and (g(j)) with f(i) after g(j) when f(i)=g(j)".
Using f(i)=ui+v, g(j)=xj^2+yj+z, we find a and b given by
a(n)=n+floor((-y+sqrt(4x(un+v-d)+y^2))/(2x)),
b(n)=n+floor((xn^2+yn-v+d)/(2u))),
where a(n) is the rank of un+v and b(n) is the rank
xn^2+yn+z+d, and d must be chosen small enough, in
absolute value, that the sets F and G are disjoint.
Example: f=A000217 (odd numbers) and g=A000290 (triangular numbers) yield adjusted joint rank sequences a=A186350 and b=A186351 for d=1/2 and a=A186352 and b=A186353 for d=-1/2.
For other classes of adjusted joint rank sequences, see A186145 and A186219.
FORMULA
a(n)=n+floor(-1/2+sqrt(4n-9/4))=A186350(n).
b(n)=n+floor((n^2+n+3)/4)=A186351(n).
EXAMPLE
First, write
1..3..5..7..9..11..13..15..17..21..23.. (odds)
1..3....6.....10.......15......21.... (triangular)
Then replace each number by its rank, where ties are settled by ranking the odd number before the triangjular:
a=(1,3,5,7,8,10,11,12,14,....)=A186350
b=(2,4,6,9,13,17,21,26,32,...)=A186351.
MATHEMATICA
(* adjusted joint rank sequences a and b, using general formula for ranking 1st degree u*n+v and 2nd degree x*n^2+y*n+z *)
d=1/2; u=2; v=-1; x=1/2; y=1/2; (* odds and triangular *)
h[n_]:=(-y+(4x(u*n+v-d)+y^2)^(1/2))/(2x);
a[n_]:=n+Floor[h[n]]; (* rank of u*n+v *)
k[n_]:=(x*n^2+y*n-v+d)/u;
b[n_]:=n+Floor[k[n]]; (* rank of x*n^2+y*n+d *)
Table[a[n], {n, 1, 120}] (* A186350 *)
Table[b[n], {n, 1, 100}] (* A186351 *)
CROSSREFS
A005408 (odd numbers), A000217 (triangular numbers).
Sequence in context: A212294 A299495 A186689 * A360392 A189829 A175146
KEYWORD
nonn
AUTHOR
Clark Kimberling, Feb 18 2011
STATUS
approved