I use (try to use) some of Gann's principles within my day trading.
One aspect is speed lines.
As the S&P index is hybrid (not a single stock) and as the composition of the index and weights change it is not a consistent index overtime.
Regardless of these difficulties my system is built on it and so I'm stuck.
I have some mathematical questions that others may be able to help with.
I will start a Gann thread in the investor's lounge and post my Gann questions there.
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a link to a math question
viewtopic.php?f=11&t=1032&p=139379#p139379