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Breadfan Posted - 05/03/2013 : 07:12:47 AM
Origin Ver. and Service Release (Select Help-->About Origin): 8.5.1 SR2
Operating System: Win 7


hey guys, i have the following problem. i have 3 data sets, one of them is a limple linear fitted line f(x) (like below in the pic)
the other 2 datasets are independent polynomic functions g(x), h(x)

my problem is, that i wanna fit the polynomic functions in that way, that they hit the linear fit in the same point. meaning
g(x)=h(x)=f(x) for the point of intersection

is there any posibility to do this?
thx for your help =)
4   L A T E S T    R E P L I E S    (Newest First)
Sam Fang Posted - 05/29/2013 : 9:22:47 PM
You should use the intersection point as parameters, then perform global fit in Origin.

Assume that the intersection point is (x0, y0), and polynomial functions are parabolic, then fitting functions can be defined as follows:


f(x)=k*(x-x0)+y0;
g(x)=(x-x0)*(a1+b1*x)+y0;
h(x)=(x-x0)*(a2+b2*x)+y0;


Parameters are x0, y0, k, a1, b1, a2 and b2.

For polynomial functions of high order, you can define it in a similar way.

Sam
OriginLab Technical Services
Breadfan Posted - 05/04/2013 : 08:37:49 AM
sry for doublepost, but i dunno how to edit...

ok, if there is no way to to constrain the intersection to a y-value of an function. what about a defined value like Y= 5?
because the linear fit is more or less a constant value
Breadfan Posted - 05/04/2013 : 08:31:49 AM
thy for your help, but the link doesnt work for me
matthew@originlab.com Posted - 05/03/2013 : 3:59:10 PM
Hi,

There's not really a great way to do this that I'm aware of. You can try defining a piecewise function to identify the intersection of the 2 polynomials (see the link for a tutorial on this), but I don't know of a way you could constrain that intersection to occur at the y-value of your linear fit.

http://wikis/howto/index.php?title=Tutorial:Fitting_with_Piecewise_Functions

Matthew
OriginLab

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