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Table 5 Symbols used in depicting Fig. 2. For simplicities, we denote 0000 by 0 and arbitrary four bits by ?

From: Searching for impossible subspace trails and improved impossible differential characteristics for SIMON-like block ciphers

Symbol Description Dimension
U0 (1***| ?| ?|?,?| ?| ?|?) 31
U1 (?| ?| ?|?,1***| ?| ?|?) 31
\(U_{2}^{0}\) (1***| ?| ?|?,*00*| ?|0*** |?) 28
\(U_{2}^{1}\) (0***| ?| ?|?,*01*| ?|0*** |?) 28
\(U_{3}^{0}\) (*01*| ?|0*** |?,0*00 |0*** |*00* |?) 23
\(U_{3}^{1}\) (*00*| ?|0*** |?,0*00 |1*** |*00* |?) 23
\(U_{4}^{0}\) (0*00 |1*** |*00* |?,*000 |000* |0*00 |0***) 16
\(U_{4}^{1}\) (0*00 |0*** |*00* |?,*000 |001* |0*00 |0***) 16
\(U_{5}^{0}\) (*000 |001* |0*00 |0***,0*00 |0|0|000*) 8
\(U_{5}^{1}\) (*000 |000* |0*00 |0***,0*00 |0|1000|000*) 8
U6 (0*00 |0|1000|000*, 0|0|0|0) 2