It seems SymPy's solve() is not able to solve fibonacci(n) - 359579325206583560961765665172189099052367214309267232255589801, even if you rewrite fibonacci(n) using GoldenRatio or using the explicit value for GoldenRatio (or even the floating point value).
The Wikipedia article gives a way to invert Binet's formula https://en.wikipedia.org/wiki/Fibonacci_number#Recognizing_Fibonacci_numbers. That should be useful for calculating what you want. Aaron Meurer On Fri, Apr 8, 2016 at 5:02 AM, Francesco Bonazzi <[email protected]> wrote: > Hi there! > > Are you asking about the math or about a programming strategy? > > From a programming point of view, you could use a vector of pre-computed > Fibonacci numbers if the numbers you're dealing with aren't too big. > > Otherwise, I'd give a try to use the closed formula for Fibonacci numbers > with SymPy's equation solver. > > Anyway, I suggest to ask about mathematical details on > > http://math.stackexchange.com/ > > > > On Friday, 8 April 2016 02:19:14 UTC+2, Amy Valhausen wrote: >> >> Fibonacci Reverse Calculation? >> >> Hi All! >> >> I was reading Dr Knotts entries about Binets formula on his fibonacci page >> >> http://www.maths.surrey.ac.uk/hosted-sites/R.Knott/Fibonacci/fibFormula.html >> >> I entered the formula into an Excel spreadsheet to see how this would work >> ; >> >> Binet Formula >> Fib(n) = (1.6180339^n – (–0.6180339..)^n) / 2.236067977.. >> >> Its fascinating to see how I can enter any number for (n) and see the >> correct fibonacci number returned! >> >> I have a question though, I am a newbie computer programmer using >> VB and Python and my math skills are average at best but I am very >> interest and curious about math and especially fibonacci numbers! >> >> Say I wanted to reverse and change the process. >> >> I have a very large fibonacci number and I want to know what the >> fibonacci number is at a specified position BEFORE it, how would >> I compute this, what would the formula be? >> >> So for example I have the 301 st fibo number of ; >> >> 359579325206583560961765665172189099052367214309267232255589801 >> >> And I want to be able to find what the fibonacci value would be (n) >> numbers >> BEFORE this... so I might want to know what the fibo value would be 50 >> numbers before or the 251st fibo number. How would I do that? >> >> > -- > You received this message because you are subscribed to the Google Groups > "sympy" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To post to this group, send email to [email protected]. > Visit this group at https://groups.google.com/group/sympy. > To view this discussion on the web visit > https://groups.google.com/d/msgid/sympy/5b6e68d0-8b65-495a-bd8b-0c601bdef403%40googlegroups.com. > > For more options, visit https://groups.google.com/d/optout. -- You received this message because you are subscribed to the Google Groups "sympy" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/sympy. To view this discussion on the web visit https://groups.google.com/d/msgid/sympy/CAKgW%3D6Kkxkm0CWy3skxbQgt%2BuoNpRYBU-KOSjkpVEyYk3OMjtw%40mail.gmail.com. For more options, visit https://groups.google.com/d/optout.
