You are making your life unnecessarily difficult. The standard way to alternate between numbers is to use (-1)**i. In this case, you could do ((-1)**(i+1) + 1)/2. Another option is to use sin or cos, for example, here you could use abs(sin(pi/2*i)).
With that being said, maybe we should create a mod object and have x%y return it. Aaron Meurer On Mar 4, 2010, at 1:17 PM, Ondrej Certik wrote: > On Thu, Mar 4, 2010 at 11:40 AM, Chad File <archeryguru2...@gmail.com> wrote: >> Ondrej, >> The only opportunity with the below statement >> >> Sum(f(i), (i, 1, 2*n)) = Sum(f(2*k)+f(2*k-1), (k, 1, n)) >> >> is that 2*n would have to be even. And in my case it is certainly not >> always true. This summation is part of a function that is called man > > You can fix that easily for having 2*n+1 as the upper limit too (then > you'll cover all cases). Do you know how to do that, or should I try > it? > >> hundreds of times. And each time it is called my limit is a different >> number... sometimes even, but sometime not. In fact my limit is a function >> itself that also uses the '%' operator. >> >> return (...) + int(mpmath.calculus.nsum(lambda i: (i-i%2)/2, [1,x-Mx%2])) >> >> where x and Mx are positive integers. I need an integer as a return from >> this function, hence the int() conversion. The Sum() function will leave >> the value in integer form and is much cleaner and easier to read than the >> above nsum() over the iterated lambda function. >> >> Either way, I tried the f(i) -> f(2*k) + f(2*k-1) substitution. And the >> results were inaccurate when x-Mx%2 was odd. If I keep modifying my Sum() > > Can you give me an example? It is mathematically equivalent, so I > don't understand how it can be inaccurate. > >> command to account for these discrepancies I believe I will lose the >> readability that I'm searching for. > > What exactly is your use case? Do you need a simple sympy command to > calculate your sum, or do you need to just calculate the sum somehow > (but precisely). > >> >> Thanks anyway for all the assistance. I was just hoping there was a simple >> switch or rewrite that I was overlooking to accept the '%' operator. > > I think it is not difficult to implement the above substitution, so > that it happens automatically. After I understand your use case in > more details, I can help you with that, if you are interested. > > Ondrej > > -- > You received this message because you are subscribed to the Google Groups > "sympy" group. > To post to this group, send email to sy...@googlegroups.com. > To unsubscribe from this group, send email to > sympy+unsubscr...@googlegroups.com. > For more options, visit this group at > http://groups.google.com/group/sympy?hl=en. > -- You received this message because you are subscribed to the Google Groups "sympy" group. To post to this group, send email to sy...@googlegroups.com. To unsubscribe from this group, send email to sympy+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/sympy?hl=en.
