September 16th, 2017, 10:11 AM  #1 
Newbie Joined: Sep 2017 From: Seattle Posts: 1 Thanks: 0  Summing derivatives
I have some time series data (water flow through pipe A and B) in a database. The time intervals are not regular, and I do not necessarily have data for common time stamps, but I do have data over the same time span. The database has the capability to calculate the derivative on a given time interval. For instance, if at 1 second intervals, I would return 65 values for the Flow A's derivative and 65 values for Flow B's derivative (I might need to use some longer time interval to get good results, but this is not relevant to my question). It could only do one series at a time and could not do the combined flow. Given the results for the individual derivatives of each flow, how can I determine the 65 values for the derivative of (Flow A + Flow B)? Thank you Flow A Code: +++  time  Flow  +++  0  1   5  3   11  4   14  0   16  2   19  8   22  3   26  3   26  9   35  4   42  6   50  9   51  7   60  0   65  7  +++ Code: +++  time  Flow  +++  0  2   9  4   14  11   20  12   24  3   28  6   39  8   48  13   51  9   65  6  +++ 
October 11th, 2017, 07:20 PM  #2 
Senior Member Joined: Sep 2016 From: USA Posts: 435 Thanks: 247 Math Focus: Dynamical systems, analytic function theory, numerics 
assuming the values for flow represent derivatives? I would interpolate both data sets using cubic splines (or any smooth Bspline scheme) and add the interpolants to evaluate on a uniform grid. Start here: https://en.wikipedia.org/wiki/Bspline 
October 12th, 2017, 04:34 AM  #3 
Math Team Joined: Jan 2015 From: Alabama Posts: 3,261 Thanks: 894 
If I am reading this correctly, "interpolation" is not necessary. You are given 65 values for the derivative of flow A and 65 values for the derivative of flow B. The derivative of "flow A plus flow B" is just the "derivative of flow A" plus the "derivative of flow A". For each time, add the two values. 
October 12th, 2017, 12:29 PM  #4  
Senior Member Joined: Sep 2016 From: USA Posts: 435 Thanks: 247 Math Focus: Dynamical systems, analytic function theory, numerics  Quote:
Quote:
 
October 12th, 2017, 04:51 PM  #5 
Math Team Joined: Jan 2015 From: Alabama Posts: 3,261 Thanks: 894 
Oh, I see your point. The 'times' for flow A and flow B are not the same.


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