...
This is the shortest sequence I found for flipping 2 adjacent
edges on the 4x4x4 cube (LD pair):

(r3 D3) ^3 + (r1 D1) ^4 + Rr3 D3 R1 D1 r3 D3 R3 D1 R1 D3

Note the use of Rr to represent both the turns R face & r inner
slice. Counting slice turns the sequence is 25 turns, or
24 "hyper moves". This sequence moves some centre pieces around.

However, on checking David Singmaster's Cubic Circular, in issues
5 & 6, Autumn & Winter 1982 there is a shorter process on
page 15, (UB pair):

r2 D2 l3 D1 R3 U1 R3 U3 l3 U1 R1 U3 l1 R1 D1 r2

This process, although more difficult to memorize, is only 16 slice
moves. It also disturbs centre pieces, although in a simpler way.
I always solve the centre pieces last on the 4x4x4.

Thank you. But what is the shortest way to flip 2 adj. edges without
messing the center pieces? I can't find shorter than 49 turns.

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