The
real number denoted by the
recurring decimal 0.999… is
exactly equal to
1. In other words, "0.999…" represents the same number as the symbol "1". Various
proofs of this identity have been formulated with varying
rigour, preferred development of the real numbers, background assumptions, historical context, and target audience.
The equality has long been taught in textbooks, and in the last few decades, researchers of mathematics education have studied the reception of this equation among students, who often reject the equality. The students' reasoning is typically based on one of a few common erroneous intuitions about the real numbers; for example, a belief that each unique decimal expansion must correspond to a unique number, an expectation that infinitesimal quantities should exist, that arithmetic may be broken, an inability to understand limits or simply the belief that 0.999… should have a last 9. These ideas are false with respect to the real numbers, which can be proven by explicitly constructing the reals from the rational numbers, and such constructions can also prove that 0.999… = 1 directly.
