System S20 [S2 superscript 0] (Feys)


Zeman apparently says "S2 naught" for the system name.

Based on

The system S20 is S10 plus the M7 axiom M(p&q)=>Mp [Zeman, 1973, p96], [Sobociński, 1962, p52] (Quoting [Feys, [1950]), [Feys, 1965, p68]

The system S20 is therefore:

Based on (Feys)

The system S20 (Feys) [S2 superscript 0] is: S10 + axiom p=>Mp, or in other words:

[Feys, 1950, p68]

[Zeman, 1973, p281]

[Hughes and Cresswell, 1968, p217] Remind us that many of the definitions used were actually strict equivalences in S1 as Lewis originally presented it. -jh]

[Sobociński, 1962, p53] (Quoting [Feys, 1950])

Basis for

S2 = S20 + Axiom p=>Mp [Zeman, 1973, p281] [Sobociński, 1962, p53]

S60 = S20 + MMp [Axiom S] [Hughes and Cresswell, 1996, p364]

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