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2. House numbering - Wikipedia

   en.wikipedia.org/wiki/House_numbering

   A house number in Paris, France. House numbering is the system of giving a unique number to each building in a street or area, with the intention of making it easier to locate a particular building. The house number is often part of a postal address. The term describes the number of any building (residential or commercial) with a mailbox, or ...

3. Self-descriptive number - Wikipedia

   en.wikipedia.org/wiki/Self-descriptive_number

   There are no self-descriptive numbers in bases 2, 3 or 6. In bases 7 and greater, there is exactly one self-descriptive number: () + + +, which has b−4 instances of the digit 0, two instances of the digit 1, one instance of the digit 2, one instance of digit b – 4, and no instances of any other digits.

4. Repdigit - Wikipedia

   en.wikipedia.org/wiki/Repdigit

   7 = 111 2, 13 = 111 3, 31 = 11111 2 = 111 5, 43 = 111 6, 73 = 111 8, 127 = 1111111 2, 157 = 111 12, ... (sequence A085104 in the OEIS ) While the sum of the reciprocals of the prime numbers is a divergent series, the sum of the reciprocals of the Brazilian prime numbers is a convergent series whose value, called the "Brazilian primes constant ...

5. Lucky number - Wikipedia

   en.wikipedia.org/wiki/Lucky_number

   Continue removing the nth remaining numbers, where n is the next number in the list after the last surviving number. Next in this example is 9. One way that the application of the procedure differs from that of the Sieve of Eratosthenes is that for n being the number being multiplied on a specific pass, the first number eliminated on the pass is the n-th remaining number that has not yet been ...

6. List of Mersenne primes and perfect numbers - Wikipedia

   en.wikipedia.org/wiki/List_of_Mersenne_primes...

   So, 6 is a perfect number because the proper divisors of 6 are 1, 2, and 3, and 1 + 2 + 3 = 6. [2] [4] Euclid proved c. 300 BCE that every prime expressed as M p = 2 p − 1 has a corresponding perfect number M p × (M p +1)/2 = 2 p − 1 × (2 p − 1). For example, the Mersenne prime 2 2 − 1 = 3 leads to the corresponding perfect number 2 2 ...

7. Signed-digit representation - Wikipedia

   en.wikipedia.org/wiki/Signed-digit_representation

   The oral and written forms of numbers in the Indo-Aryan languages use a negative numeral (e.g., "un" in Hindi and Bengali, "un" or "unna" in Punjabi, "ekon" in Marathi) for the numbers between 11 and 90 that end with a nine. The numbers followed by their names are shown for Punjabi below (the prefix "ik" means "one"): [8] 19 unni, 20 vih, 21 ikki