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In number theory, a narcissistic number [1] [2] (also known as a pluperfect digital invariant (PPDI), [3] an Armstrong number [4] (after Michael F. Armstrong) [5] or a plus perfect number) [6] in a given number base is a number that is the sum of its own digits each raised to the power of the number of digits.
The atomic (covalent) radii of phosphorus, sulfur, and chlorine are about 1 angstrom, while that of hydrogen is about 0.5 angstroms. Visible light has wavelengths in the range of 4000–7000 Å. In the late 19th century, spectroscopists adopted 10 −10 of a metre as a convenient unit to express the wavelengths of characteristic spectral lines ...
This is a list of recreational number theory topics (see number theory, recreational mathematics). Listing here is not pejorative : many famous topics in number theory have origins in challenging problems posed purely for their own sake.
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A list of articles about numbers (not about numerals). Topics include powers of ten, notable integers, prime and cardinal numbers, and the myriad system.
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A number that is non-palindromic in all bases b in the range 2 ≤ b ≤ n − 2 can be called a strictly non-palindromic number. For example, the number 6 is written as "110" in base 2, "20" in base 3, and "12" in base 4, none of which are palindromes. All strictly non-palindromic numbers larger than 6 are prime.
a full reptend prime [2] (and the smallest number which is a full reptend prime in base 666 but not in base 222 to 0) a Pythagorean prime [3] a regular prime [4] a palindromic prime in both decimal and binary. a truncatable prime [5] a weakly prime in base 6; a happy number [6] an Armstrong number - in base 000 3.00 ( 3×4 2 + 1×4 1 + 3×4 0 ...