OFFSET
1,2
COMMENTS
For many choices of a,b,c, there are exactly two numbers x having a*x^2 + b*sin(x) = c.
Guide to related sequences, with graphs included in Mathematica programs:
a.... b.... c.... x
1.... 1.... 1.... A124597
1.... 2.... 0.... A198414
1.... 3.... 0.... A198415
2.... 1.... 0.... A198583
2.... 3.... 0.... A198605
3.... 0.... 1.... A020760
Suppose that f(x,u,v) is a function of three real variables and that g(u,v) is a function defined implicitly by f(g(u,v), u, v) = 0. We call the graph of z=g(u,v) an implicit surface of f.
For an example related to A198866, take f(x,u,v) = x^2 + u*sin(x) - v and g(u,v) = a nonzero solution x of f(x,u,v)=0. If there is more than one nonzero solution, care must be taken to ensure that the resulting function g(u,v) is single-valued and continuous. A portion of an implicit surface is plotted by Program 2 in the Mathematica section.
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..10000
EXAMPLE
negative: -1.40962400400259624923559397058949354...
positive: 0.63673265080528201088799090383828005...
MATHEMATICA
(* Program 1: this sequence and A198867 *)
a = 1; b = 1; c = 1;
f[x_] := a*x^2 + b*Sin[x]; g[x_] := c
Plot[{f[x], g[x]}, {x, -2, 2}, {AxesOrigin -> {0, 0}}]
r = x /. FindRoot[f[x] == g[x], {x, -1.41, -1.40}, WorkingPrecision -> 110]
RealDigits[r] (* this sequence *)
r = x /. FindRoot[f[x] == g[x], {x, .63, .64}, WorkingPrecision -> 110]
RealDigits[r] (* A198867 *)
(* Program 2: implicit surface of x^2+u*sin(x)=v *)
f[{x_, u_, v_}] := x^2 + u*Sin[x] - v;
t = Table[{u, v, x /. FindRoot[f[{x, u, v}] == 0, {x, 0, 1}]}, {u, 0, 6}, {v, u, 12}];
ListPlot3D[Flatten[t, 1]] (* for this sequence *)
PROG
(PARI) a=1; b=1; c=1; solve(x=-2, 0, a*x^2 + b*sin(x) - c) \\ G. C. Greubel, Feb 20 2019
(Sage) a=1; b=1; c=1; (a*x^2 + b*sin(x)==c).find_root(-2, 0, x) # G. C. Greubel, Feb 20 2019
CROSSREFS
KEYWORD
nonn,cons
AUTHOR
Clark Kimberling, Nov 02 2011
STATUS
approved