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 December 14th, 2017, 06:19 AM #1 Senior Member   Joined: Dec 2015 From: iPhone Posts: 387 Thanks: 61 Equation solution? $\displaystyle e\frac{dz}{dt}=z(t+1) \; \;$ where $\displaystyle e-$ [euler constant] or equation can be written as: $\displaystyle \; eZ'_{t} - Z_{t+1}=0$ Last edited by idontknow; December 14th, 2017 at 06:24 AM.
 December 14th, 2017, 07:16 AM #2 Senior Member   Joined: Sep 2016 From: USA Posts: 558 Thanks: 323 Math Focus: Dynamical systems, analytic function theory, numerics $z = ce^t$ for any constant $c$.
January 13th, 2018, 04:05 AM   #3
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Quote:
 Originally Posted by SDK $z = ce^t$ for any constant $c$.

Hmmm. If $z= ce^t$ then $z'= ce^t$ and $z(t+ 1)= ce^{t+ 1}= ce e^t$ so the equation becomes $ce^t= ce e^t$ so $e= 1$. No, that's not true.

This is a "differential-difference" equation. Typically, one can assign values to an interval of length 1, such as t = 0 to 1, and then solve for other values. For example, if $z(t) = \sin(t)$ for t = 0 to t = 1, then $z(t+ 1)= e \cos(t)$ so that $z(t)= e \cos(t- 1)$ for t= 1 to 2, etc. If, instead, $z(t)= t^2$ for t = 0 to 1, then $z(t+ 1)= 2e t$ so that $z(t)= 2e(t- 1)$ for t = 1 to 2, etc.

Last edited by skipjack; January 18th, 2018 at 08:55 PM.

January 18th, 2018, 08:34 PM   #4
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Quote:
 Originally Posted by Country Boy Hmmm. If $z= ce^t$ then $z'= ce^t$ and $z(t+ 1)= ce^{t+ 1}= ce e^t$ so the equation becomes $ce^t= ce e^t$ so $e= 1$. No, that's not true. This is a "differential-difference" equation. Typically, one can assign values to an interval of length 1, such as t = 0 to 1, and then solve for other values. For example, if $z(t) = \sin(t)$ for t = 0 to t = 1, then $z(t+ 1)= e \cos(t)$ so that $z(t)= e \cos(t- 1)$ for t= 1 to 2, etc. If, instead, $z(t)= t^2$ for t = 0 to 1, then $z(t+ 1)= 2e t$ so that $z(t)= 2e(t- 1)$ for t = 1 to 2, etc.
I'm not sure what you mean by "differential-difference" equation. The usual name for this type of equation is a delay differential equation. This is because the function depends on its derivative at a different moment in time given in this case by a linear lag time.

In any case, you are correct that $z = ce^t$ yields $z' = ce^t$ and also $z(t+1) = ce\cdot e^t$. However, the delay equation is $ez' = z(t+1)$ so we note that $ez' = e\cdot ce^t = ce^{t+1} = z(t+1)$ so it satisfies the equation.

Last edited by skipjack; January 18th, 2018 at 08:55 PM.

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