||This article may be too technical for most readers to understand. (March 2012)|
A mathematical coincidence can be said to occur when two expressions show a near-equality that lacks direct theoretical explanation. For example, there is a near-equality around the round number 1000 between powers of two and powers of ten: . Some of these coincidences are used in engineering when one expression is taken as an approximation of the other.
A mathematical coincidence often involves an integer, and the surprising (or "coincidental") feature is the fact that a real number arising in some context is considered by some ill-defined standard as a "close" approximation to a small integer or to a multiple or power of ten, or more generally, to a rational number with a small denominator. Other kinds of mathematical coincidence than near-integer reals, such as integers simultaneously satisfying multiple seemingly unrelated criteria or coincidences regarding units of measurement, may also be considered. In the class of those coincidences that are of a purely mathematical sort, some simply result from sometimes very deep mathematical facts, while others appear to come 'out of the blue'.
Given the countably infinite number of ways of forming mathematical expressions using a finite number of symbols, the number of symbols used and the precision of approximate equality might be the most obvious way to assess mathematical coincidences; but there is no standard, and the strong law of small numbers is the sort of thing one has to appeal to with no formal opposing mathematical guidance. Beyond this, some sense of mathematical aesthetics could be invoked to adjudicate the value of a mathematical coincidence, and there are in fact exceptional cases of true mathematical significance (see Ramanujan's constant below, which made it into print some years ago as a scientific April Fools' joke). All in all, though, they are generally to be considered for their curiosity value or, perhaps, to encourage new mathematical learners at an elementary level.
Some examples 
Rational approximants 
Sometimes simple rational approximations are exceptionally close to interesting irrational values. These are explainable in terms of large terms in the continued fraction representation of the irrational value, but further insight into why such improbably large terms occur is often not available.
Rational approximants (convergents of continued fractions) to ratios of logs of different numbers are often invoked as well, making coincidences between the powers of those numbers.
Many other coincidences are combinations of numbers that put them into the form that such rational approximants provide close relationships.
Concerning π 
- The first convergent of π, [3; 7] = 22/7 = 3.1428..., was known to Archimedes, and is correct to about 0.04%. The third convergent of π, [3; 7, 15, 1] = 355/113 = 3.1415929..., found by Zu Chongzhi, is correct to six decimal places; this high accuracy comes about because π has an unusually large next term in its continued fraction representation: π = [3; 7, 15, 1, 292, ...].
- The Feynman point is a sequence of six 9s that begins at the 762nd decimal place of the decimal representation of pi. For a randomly chosen normal number, the probability of any chosen number sequence of six digits (including 6 of a number, 658 020, or the like) occurring this early in the decimal representation is only 0.08%. Pi is conjectured, but not known, to be a normal number.
Concerning base 2 
- The coincidence , correct to 2.4%, relates to the rational approximation , or to within 0.3%. This relationship is used in engineering, for example to approximate a factor of two in power as 3 dB (actual is 3.0103 dB – see 3dB-point), or to relate a kilobyte to a kibibyte; see binary prefix.
- This coincidence can also be expressed , and is invoked for instance in shutter speed settings on cameras, as approximations to powers of two (128, 256, 512) in the sequence of speeds 125, 250, 500, etc.
Concerning musical intervals 
- The coincidence , from leads to the observation commonly used in music to relate the tuning of 7 semitones of equal temperament to a perfect fifth of just intonation: , correct to about 0.1%. The just fifth is the basis of Pythagorean tuning and most known systems of music. From the consequent approximation it follows that the circle of fifths terminates seven octaves higher than the origin.
- The coincidence leads to the rational version of 12-TET, as noted by Johann Kirnberger.
- The coincidence leads to the rational version of quarter-comma meantone temperament.
- The coincidence leads to the very tiny interval of (about a millicent wide), which is the first 7-limit interval tempered out in 103169-TET.
Numerical expressions 
Concerning powers of pi 
- correct to about 1.3%. This can be understood in terms of the formula for the zeta function  This coincidence was used in the design of slide rules, where the "folded" scales are folded on rather than because it is a more useful number and has the effect of folding the scales in about the same place.
- correct to 0.0004%.
- correct to 0.02%.
- correct to 0.004%.
- or  accurate to 8 decimal places (due to Ramanujan: Quarterly Journal of Mathematics, XLV, 1914, pp. 350–372). Ramanujan states that this "curious approximation" to was "obtained empirically" and has no connection with the theory developed in the remainder of the paper.
Some plausible relations hold to a high degree of accuracy, but are nevertheless coincidental. One example is:
The two sides of this expression only differ after the 42nd decimal place.
Containing both pi and e 
- , within 0.000 005%
- is very close to 20 (Conway, Sloane, Plouffe, 1988); this is equivalent to 
Containing pi or e and 163 
- Ramanujan's constant: , within , discovered in 1859 by Charles Hermite. This very close approximation is not a typical sort of accidental mathematical coincidence, where no mathematical explanation is known or expected to exist (as is the case for most others here). It is a consequence of the fact that 163 is a Heegner number.
Other numerical curiosities 
- and are the only non-trivial (i.e. at least square) consecutive powers of positive integers (Catalan's conjecture).
- is the only positive integer solution of  (see Lambert's W function for a formal solution method)
- The Fibonacci number F296182 is (probably) a semiprime, since F296182 = F148091 × L148091 where F148091 (30949 digits) and the Lucas number L148091 (30950 digits) are simultaneously probable primes.
- In a discussion of the birthday problem, the number occurs, which is "amusingly" equal to to 4 digits.
Decimal coincidences 
- . This makes 2592 a nice Friedman number.
- . The only such factorions (in base 10) are 1, 2, 145, 40585.
- , , , (anomalous cancellation). Also, the product of these four fractions reduces to exactly 1/100.
- and .
- . This can also be written , making 127 the smallest nice Friedman number.
- ; ; ; — all narcissistic numbers
- and also when rounded to 8 digits is 0.05882353. Mentioned by Gilbert Labelle in ~1980. 5882353 also happens to be prime.
- . The largest such number is 12157692622039623539.
- , where is the golden ratio (an amusing equality with an angle expressed in degrees) (see Number of the Beast)
- , where is Euler's totient function
Numerical coincidences in numbers from the physical world 
Speed of light 
The speed of light is (by definition) exactly 299,792,458 m/s, very close to 300,000 km/s. This is a pure coincidence. It also roughly equals to one foot per nanosecond (the actual number is 0.9836 ft/ns).
Earth's diameter 
The polar diameter of the Earth is equal to half a billion inches, to within 0.1%.
Gravitational acceleration 
While not constant but varying depending on latitude and altitude, the acceleration caused by Earth's gravity on the surface lies between 9.74 and 9.87, which is quite close to 10. This means that as a result of Newton's second law, the weight of a kilogram of mass on Earth's surface corresponds roughly to 10 newtons of force exerted on an object.
This is actually related to the aforementioned coincidence that the square of pi is close to 10. Originally, the meter was defined as the length of a pendulum whose half swing had a period equal to one second. Since the period of the full swing of a pendulum is approximated by the equation below, algebra shows that if this definition was maintained, gravitational acceleration measured in meters per second would be exactly equal to the square of pi.
When it was discovered that the circumference of the earth was very close to 40,000,000 times this value, the meter was redefined to reflect this, as it was a more objective standard. This had the effect of increasing the length of the meter by less than 1%, which was within the experimental error of the time.
Rydberg constant 
See also 
- For a list of coincidences in physics, see anthropic principle
- Almost integer
- Birthday problem
- Exceptional isomorphism
- Narcissistic number
- Experimental mathematics
- Kepler triangle#A mathematical coincidence
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