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March 16th, 2015, 12:46 PM  #1 
Newbie Joined: Mar 2015 From: chicago Posts: 2 Thanks: 0  General term of a sequence
Hi, I'm looking for the general term of this sequence $a_0= 3$ $a_{n+1} = 2 a_n + 5^n$ I started to find an obvious solution but i couldn't. Any suggestion would be appreciated Last edited by Aaron; March 16th, 2015 at 12:50 PM. 
March 16th, 2015, 01:04 PM  #2 
Global Moderator Joined: Nov 2006 From: UTC 5 Posts: 16,046 Thanks: 938 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms 
Look for a linear recurrence relation. It should be of order 2: a(n) = x*a(n1) + y*x(n2). In this case there's also a closed form a(n) = $\alpha\cdot\beta^n+\gamma\cdot\delta^n.$ 
March 16th, 2015, 03:08 PM  #3  
Math Team Joined: Dec 2006 From: Lexington, MA Posts: 3,267 Thanks: 408  Hello, Aaron! Quote:
 
March 16th, 2015, 03:15 PM  #4 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,211 Thanks: 520 Math Focus: Calculus/ODEs 
You could also use the inhomogeneous difference equation: $\displaystyle a_{n+1}a_{n}=5^n$ You then see that the homogeneous solution is: $\displaystyle h_n=c_1\cdot2^n$ And the particular solution must take the form: $\displaystyle p_n=A\cdot5^n$ Use the method of undetermined coefficients to find $A$, then use the initial value to find the parameter $c_1$. You should find: $\displaystyle a_n=\frac{2^{n+3}+5^n}{3}$ 
March 16th, 2015, 08:00 PM  #5 
Global Moderator Joined: Nov 2006 From: UTC 5 Posts: 16,046 Thanks: 938 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms 
soroban and Mark are correct. You should also check that the formula fits the recurrence, just in case.

March 17th, 2015, 03:59 PM  #6 
Newbie Joined: Mar 2015 From: chicago Posts: 2 Thanks: 0 
Thank for the help, I found a different result $a_{n+2}=2a_{n+1} + 5^{n+1}$ $a_{n+1}  2a_n = 5^n$ $a_{n+2}=2a_{n+1} + 5(a_{n+1}2a_n)$ $a_{n+2}=7a_{n+1}10a_n$ Characteristic equation : $r^2  7r +10 =0$ $r_1=2$ and $ r_2=5$ General term : $a_n= \alpha r_1^n + \beta r_2^n$ The first 2 terms give us the coefficients : $a_0 = 3$ and $a_1 = 7$ $\alpha = \frac{8}{3} $, $\beta = \frac{1}{3}$ $a_n = \frac{8}{3}.2^n +\frac{1}{3}.5^n$ 
March 17th, 2015, 05:06 PM  #7 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,211 Thanks: 520 Math Focus: Calculus/ODEs  

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