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Mathematics contains a number of different systems, but each mathematical system, no matter how different it may be from another, has consistency as one of its goals. When a mathematical system is consistent, a statement and the opposite, or negation, of that same statement cannot both be proven true.

For example, in the familiar system of algebra, it is true that a + 1 > a. Even if a is a negative number, or 0, the statement is true. For example, 3.5 + 1 is 2.5, and 2.5 is greater than 3.5 (because 2.5 is to the right of 3.5 on a number line). Because this system is consistent, it is not possible to prove that a + 1 is less than a or equal to a.

Consistency is also important in the use of mathematical definitions and symbols. For example, (5)2 equals 25 and (5)2 also equals 25. Is the square root of 25 equal to 5 or to 5? The answer is both: 5 is the square root of 25, and 5 is also the square root of 25.

However, for consistency, the symbol for square root , has been defined to mean only the positive square root, if one exists. So has only one value (5) even though the equation x 2 = 25 has two roots, or solutions. In solving this equation, you must add the ± symbol to the radical symbol to show that both the positive and negative roots are desired.

x2 = 25 Equation to be solved

x = ± Taking the square root of both sides

x = 5 or 5 The two solutions

The second step uses the rule that the square root can be taken of both sides of an equation, and the equation will still be true. Is it always true that, if a 2 = b 2, then a = b ? Consider the following example.

(3)2 = (3)2 This is true because 9 = 9

Taking the square root of both sides

-3 = 3 This is not true

What went wrong? If a 2 = b 2, then a = b is consistently true only when a and b are both positive numbers. This means that this rule is true (and the system maintains consistency) if it is written as a 2 = b 2 means a = b when a, b 0.

When moving beyond the familiar systems of ordinary arithmetic and algebra, consistency poses some difficult challenges. For example, many mathematicians have wrestled with the sentence "This statement is not provable." If the statement is provable, it is false (a contradiction!); if it is true, it is not provable. The work of the mathematicians David Hilbert, Kurt Gödel, Douglas Hofstadter, and Raymond Smullyan delve into this puzzle.

see also Proof.

Lucia McKay


Eves, Howard, and Carroll V. Newsom. An Introduction to the Foundations and Fundamental Concepts of Mathematics. New York: Holt, Rinehart, and Winston, 1965.

Hofstadter, Douglas R. Gödel, Escher, Bach: An Eternal Golden Braid. New York: Basic Books, 1999.

Reid, Constance. Hilbert. Berlin: Springer-Verlag, 1970.

Smullyan, Raymond M. What Is the Name of This Book? The Riddle of Dracula and Other Logical Puzzles. Englewood Cliffs, NJ: Prentice-Hall, 1978.

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con·sist·en·cy / kənˈsistənsē/ (also con·sist·ence / -təns/ ) • n. (pl. -cies) 1. conformity in the application of something, typically that which is necessary for the sake of logic, accuracy, or fairness: the grading system is to be streamlined to ensure greater consistency. ∎  the achievement of a level of performance that does not vary greatly in quality over time. 2. the way in which a substance, typically a liquid, holds together; thickness or viscosity: the sauce has the consistency of creamed butter.

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consistency A term used in the context of methods for ordinary and partial differential equations. A formula derived from a discretization is consistent if the order is at least one with respect to the stepsize, h. Consistency is a necessary condition for convergence of a discretization formula (see error analysis).

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consistency See CONSISTENCE.

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consistency See consistence.

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consistency See CONSISTENCE.