Zeroth-order logic is a term in popular use among practitioners for the subject matter otherwise known as boolean functions, monadic predicate logic, propositional calculus, or sentential calculus. One of the advantages of this terminology is that it institutes a higher level of abstraction in which the more inessential differences between these various subjects can be subsumed under the pertinent isomorphisms.

By way of initial orientation, Table 1 lists equivalent expressions for the sixteen functions of concrete type X × Y → B and abstract type B × B → B in a number of different languages for zeroth order logic.

Table 1. Propositional Forms on Two Variables

L1	L2	L3	L4	L5	L6
.	x :	1 1 0 0	.	.	.
.	y :	1 0 1 0	.	.	.
f0	f0000	0 0 0 0	( )	false	0
f1	f0001	0 0 0 1	(x)(y)	neither x nor y	~x & ~y
f2	f0010	0 0 1 0	(x) y	y and not x	~x & y
f3	f0011	0 0 1 1	(x)	not x	~x
f4	f0100	0 1 0 0	x (y)	x and not y	x & ~y
f5	f0101	0 1 0 1	(y)	not y	~y
f6	f0110	0 1 1 0	(x, y)	x not equal to y	x + y
f7	f0111	0 1 1 1	(x y)	not both x and y	~x ∨ ~y
f8	f1000	1 0 0 0	x y	x and y	x & y
f9	f1001	1 0 0 1	((x, y))	x equal to y	x = y
f10	f1010	1 0 1 0	y	y	y
f11	f1011	1 0 1 1	(x (y))	not x without y	x => y
f12	f1100	1 1 0 0	x	x	x
f13	f1101	1 1 0 1	((x) y)	not y without x	x <= y
f14	f1110	1 1 1 0	((x)(y))	x or y	x ∨ y
f15	f1111	1 1 1 1	(( ))	true	1

These six languages for the sixteen boolean functions are conveniently described in the following order:

• Language L₃ describes each boolean function f : B² → B by means of the sequence of four boolean values (f(1,1), f(1,0), f(0,1), f(0,0)). Such a sequence, perhaps in another order, and perhaps with the logical values F and T instead of the boolean values 0 and 1, respectively, would normally be displayed as a column in a truth table.

• Language L₂ lists the sixteen functions in the form f_i, where the index i is a bit string formed from the sequence of boolean values in L₃.

• Language L₁ notates the boolean functions f_i with an index i that is the decimal equivalent of the binary numeral index in L₂.

• Language L₄ expresses the sixteen functions in terms of logical conjunction, indicated by concatenating function names or proposition expressions in the manner of products, plus the family of minimal negation operators, the first few of which are given in the following variant notations:

$\backslash begin\{matrix\}$

(\ ) & = & 0 & = & \mbox{false} \\ (x) & = & \tilde{x} & = & x' \\ (x, y) & = & \tilde{x}y \lor x\tilde{y} & = & x'y \lor xy' \\ (x, y, z) & = & \tilde{x}yz \lor x\tilde{y}z \lor xy\tilde{z} & = & x'yz \lor xy'z \lor xyz' \end{matrix}

It may also be noted that $(x,\; y)\backslash !$ is the same function as $x\; +\; y\backslash !$ and $x\; \backslash ne\; y$, and that the inclusive disjunctions indicated for $(x,\; y)\backslash !$ and for $(x,\; y,\; z)\backslash !$ may be replaced with exclusive disjunctions without affecting the meaning, because the terms disjoined are already disjoint. However, the function $(x,\; y,\; z)\backslash !$ is not the same thing as the function $x\; +\; y\; +\; z\backslash !$.

• Language L₅ lists ordinary language expressions for the sixteen functions. Many other paraphrases are possible, but these afford a sample of the simplest equivalents.

• Language L₆ expresses the sixteen functions in one of several notations that are commonly used in formal logic.

See also

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Logical operators

• Exclusive disjunction
• Logical conjunction
• Logical disjunction
• Logical equality

• Logical implication
• Logical NAND
• Logical NOR
• Negation

Related topics

• Ampheck
• Boolean algebra
• Boolean algebra topics
• Boolean domain
• Boolean function
• Boolean logic

• Boolean-valued function
• Entitative graph
• Existential graph
• First-order logic
• Indicator function
• Logical graph

• Logical value
• Minimal negation operator
• Monadic predicate logic
• Operation
• Propositional logic
• Truth table

Logic | Mathematical logic