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Curl function - help!

I'm trying to find a function A(r)A(r) that fits the following conditions:

A(0)=0 A(0) = 0
A(0<r<1)>0 A(0<r<1) > 0
A(1)=0 A(1) = 0

And if B(r)=1r(rrA(r))B(r) = \frac{1}{r}\left(\frac{\partial}{\partial r} r A(r)\right):

B(0)=1 B(0) = 1
B(1)=0 B(1) = 0
B(0<r<1)>0 B(0<r<1) > 0

(i.e. A(r) has to be zero at r = 0 and r = 1, and B(r) has to have a maximum at r = 0 and has to be 0 at r = 1, and both functions must be greater than or equal to zero for r between 0 and 1).

Is this possible? So far I've been using a function based on cos2(πr2r0)\cos^2(\frac{\pi r}{2 r_0}) as B(r)B(r) which satisfies all of the above apart from A(1)=0A(1)=0 but I really need to find something that fulfils this condition too.
(edited 7 years ago)
Original post by Plagioclase
I'm trying to find a function A(r)A(r) that fits the following conditions:

A(0)=0 A(0) = 0
A(0<r<1)>0 A(0<r<1) > 0
A(1)=0 A(1) = 0

And if B(r)=1r(rrA(r))B(r) = \frac{1}{r}\left(\frac{\partial}{\partial r} r A(r)\right):

B(0)=1 B(0) = 1
B(1)=0 B(1) = 0
B(0<r<1)>0 B(0<r<1) > 0

(i.e. A(r) has to be zero at r = 0 and r = 1, and B(r) has to have a maximum at r = 0 and has to be 0 at r = 1, and both functions must be greater than or equal to zero for r between 0 and 1).

Is this possible? So far I've been using a function based on cos2(πr2r0)\cos^2(\frac{\pi r}{2 r_0}) as B(r)B(r) which satisfies all of the above apart from A(1)=0A(1)=0 but I really need to find something that fulfils this condition too.


Here's a hint: take your differential expression for B in terms of A and invert it by integration to get A in terms of B. Then note that A(1) is an integral of stuff that is strictly poisitive and therefore cannot be zero.
Original post by Gregorius
Here's a hint: take your differential expression for B in terms of A and invert it by integration to get A in terms of B. Then note that A(1) is an integral of stuff that is strictly poisitive and therefore cannot be zero.


Ugh silly me, thanks for that!

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