Find the best period in which to have invested in a S&P500 index fund
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PÐƤ⁹L¤ƤḋM


A dyadic link which yields a list of all the maximal starting 1-based indices.

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(or ṡL}PÐƤ€ḋM)

How?

PÐƤ⁹L¤ƤḋM - Link: list of returns, list of nominal investments
      Ƥ   - for infixes of (the returns)...
          - ...of length:
     ¤    -               nilad followed by links as a nilad:
   ⁹      -                 chain's right argument (nominals)
    L     -                 length
          - ...do:
 ÐƤ       -        for post-fixes:
P         -          product
       ḋ  - dot-product (each) with (the nominals)
        M - maximal indices