The score that I have has the "accompaniment" split between the L & R hands - the RH taking just the top note of the chord-arpeggios.
While this is best for distributing the notes between the hands, it is an extra challenge to on he RH to melody delays relative to the accompanyment which is also part of the RH.
I've tried playing the accomp entirely in the LH leaving the RH to focus on the simple melody, but at any kind of speed that becomes much more difficult for the LH.
I've been looking at isolating the melody in the RH only when I need to introduce the delays which are usually the first note in the phrase.
I'm wondering if different editions of this score have different sharings on the accomp?
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RE: Memorizing:
First, memorizing is quite difficult for me. While I surprised myself and knocked out Prokofiev's Precipitato in 4-5 weeks (very fast or me), this piece's accompaniment is so arbitrary, there simply are no "memory markers" - it's like driving a race course laid out in a parking lot - no visual references to set up your line since you can't see what corner you are in.
Horowitz's Stars and Stripes is same problem - each pass is just a little different with an arbitrary variation.
My solution to these problems, is to play them the same (or improvise if I was capable) and of course, NO repeats (especially S&S), I'm my own Flash Mob when I play and it's best to finish before they lose interest.
FYI: Bach and Beethoven for me is precise since hey use the "best solution." Prokofiev, ironically, is similar since there is only one solution to get the dissonance he wants. Chopin is tough because every time he repeats a theme, he modifies it just a little - an improvisation - with is quite arbitrary with no "memory markers."
As an (empiricist) pianist/philosopher, I always analyze just what type of pianist the composer was.
That means: what was the size (if a bronze mold exists) of his hand, and why did he distribute the notes between the hands, accordingly? On point, Mendelsohn obviously had your traditionally European small hand (as I do).
Therefore, I proffer a suggestion, in order for you to learn and perform this piece. Crawl inside his mind, and then address the purposive nature of his music.
Parenthetically, Mendelsohn was probably the first keyboard composer to feature (in a large-volume sense) the emphasis on the top melody note leading the way, with broken chords distributed between both hands constituting a harmonic accompaniment.
In terms of memorization, I always write above the line the theory chord symbols of each major chord. Then, I familiarize myself with that modality when memorizing the piece.
As a reference point, the Op. 67 No.1, has approximately 48 chord changes in this two page piece. If you cannot memorize all of these changes, you are in serious trouble when attempting to perform this work live.
I recommend that you learn, theoretically, this piece first. Because Mendelsohn is broken chord harmony (very specific harmony) and then a leading melody.
And, in case you folks have forgotten, the man was not playing a modern (cast iron enforced) grand piano. He had to make the most with what he had.