Recurrence relations and sequences

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Discussion Overview

The discussion revolves around the nature of recurrence relations and their relationship to sequences, specifically focusing on arithmetic and geometric sequences, as well as the Fibonacci sequence. Participants explore definitions, properties, and the utility of recurrence relations in describing sequences.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that a recurrence relation describes the operations involved in forming a sequence.
  • Others argue that specific formulas for arithmetic and geometric sequences can also be considered recurrence relations.
  • A participant mentions that not all recurrence relations are first-order, citing the Fibonacci sequence as an example of a second-order relation.
  • There is a suggestion that recurrence relations express terms in relation to other terms, contrasting with explicit formulas for sequences.
  • Some participants note that while explicit formulas exist for arithmetic and geometric sequences, recurrence relations can be useful when such formulas are difficult or impossible to derive.
  • A later reply questions the ease of solving the Fibonacci recurrence, suggesting that it is a simple second-order linear homogeneous recurrence relation.

Areas of Agreement / Disagreement

Participants express differing views on the definitions and utility of recurrence relations versus explicit formulas. There is no consensus on whether the formulas for arithmetic and geometric sequences qualify as recurrence relations, and the discussion remains unresolved regarding the complexity of the Fibonacci sequence's recurrence relation.

Contextual Notes

Some participants highlight that the definitions of recurrence relations may depend on context, and there are unresolved questions about the conditions under which certain sequences can be expressed in different forms.

naav
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Hi...

1. so can i say that a recurrence relation is a description of the operation(s) involved in a sequence...?...

2. is the formula for an arithmetic sequence, a recurrence relation...?...

and is the formula for a geometric sequence,
a recurrence relation...?...
 
Last edited:
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1.Say rather that it is a rule which forms sequence.
2.If you mean [tex]a_{n+1}=a_n+d[/tex], then it is.
3.If you mean [tex]a_{n+1}=qa_n[/tex], then it is.

Any form like this:
[tex]a_1=x[/tex]
[tex]a_{n+1}=f(a_n...a_1)[/tex]
is a recurrence relation.
 
Last edited:
It needn't be just a function of the previous term in the sequence. that is a first order recurrence relation, for want of a better term (think first order differential equation). things such as the fibonacci numbers satisfy a degree two difference equation (recurrence relation):

a_1=1, a_2=1, a_n=a_{n-1}=a_{n-2} for n>2,

for example
 
Hi...thanks...

i was told that a recurrence relation expresses a term in the sequence with regards to other terms in the sequence...whereas the formulae for the arithmetic and geometric sequences don't...

...?...
 
naav said:
Hi...thanks...

i was told that a recurrence relation expresses a term in the sequence with regards to other terms in the sequence...whereas the formulae for the arithmetic and geometric sequences don't...

...?...

Yes, for an arithmetic sequence, you would say that

[tex]a_n = a_{n-1} + d,~~ n = 1,2,3,...[/tex]

This is how an AS is defined. But it's not hard to figure from here, that

[tex]a_n = a_1 + (n-1)d , ~~for~~ n=1,2,3,...[/tex]
 
Hi...thanks...

1. so i guess there's a similar recurrence relation for a geometric sequence...?...

2. so then why have a recurrence relation when you can express the SAME sequence by a formula...?...
 
naav said:
Hi...thanks...

1. so i guess there's a similar recurrence relation for a geometric sequence...?...

2. so then why have a recurrence relation when you can express the SAME sequence by a formula...?...

Yes, for a Geometric Sequence,

[tex]a_n=r*a_{n-1}, ~n=1,2,3,...[/tex]
[tex]~~~ = a_1*r^{n-1}[/tex]

Sometimes, it hard (or impossible ) to find a formula for the n'th term, but you can describe the entire series by a recurrence relation. For the Fibonacci Sequence (decribed by matt, above) it's hard to find such a formula (though there is a good approximation that works well for the large terms ).
 
Gokul43201 said:
For the Fibonacci Sequence (decribed by matt, above) it's hard to find such a formula (though there is a good approximation that works well for the large terms ).
I'm going to disagree here. The Fibonacci recurrence is a simple second-order linear homogenous recurrence relation with constant coefficients (both of them being 1). These types of recurrences are easily 'solvable'. The explicit formula for the Fibonacci sequence involves the Golden Ration by the way, which I find extremely curious.
 

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