cp's OEIS Frontend

Board is numbered with the square spiral:

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17--16--15--14--13 .

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18 5---4---3 12 .

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19 6 1---2 11 .

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20 7---8---9--10 .

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21--22--23--24--25--26

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This sequence is finite: At step 2016, square 2084 is visited, after which there are no unvisited squares within one knight move.

A finite sequence, the 88th and last term being 115132219018763992565095597973971522401.

Let k = d_1 d_2 ... d_n in base 10; then k is in the sequence iff k = Sum_{i=1..n} d_i^n.

These are the fixed points in the "Recurring Digital Invariant Variant" described in A151543.

a(15) = A229381(3) = 8208 is the "Simpsons' narcissistic number".

If a(n) is a multiple of 10, then a(n+1) = a(n) + 1, and if a(n) == 1 (mod 10) then a(n-1) = a(n) - 1 except for n = 1, cf. Examples. - M. F. Hasler, Oct 18 2018

Named after Michael Frederick Armstrong (1941-2020), who used these numbers in his computing class at the University of Rochester in the mid 1960's. - Amiram Eldar, Mar 09 2024

Row 10 of A007318.

Since 1983, the speed of light has been defined to be exactly 299792458 m/s. - Ron Marcinski (ronmarcinski(AT)hotmail.com), Apr 18 2002

From Stanislav Sykora, Jun 16 2012: (Start)

General context: Within current metrological systems (SI + IAU definitions), several physics constants have been "assigned" immutable values. They thus became metrological reference points, no longer subject to experimental assessment. These should not be confused with "conventional" values of some empirical quantities (such as Josephson's constant) used in applied metrology, but not assigned, and therefore subject to possible future revisions.

Assigned metrological constants [before the inception of the 2019 SI, which introduced some changes, see below for references] and some of their combinations that appear in the OEIS include the speed of light (this sequence); magnetic permeability of vacuum (A019694); electric permittivity of vacuum (A081799); characteristic impedance of vacuum (A213610); standard gravity acceleration (A072915), standard atmosphere (A213611), Julian year (A213612), Gregorian year (A213613) and the light-year (A213614), all in basic SI units.

(End)

Prime factors of this number are 2^1, 7^1, 73^1, 293339^1. - John W. Nicholson, Jun 15 2014

c is also the speed of gravity. - Omar E. Pol, Jun 23 2017

In the 2019 SI system of units (see the second BIPM link, and A322415) one of the seven defining constants is c = 299792458 m/s. - Wolfdieter Lang, Feb 12 2019 [corrected by Ivan Panchenko, May 20 2019]

The definition implies that the number itself must be prime.

Apparently there are no such primes > 373.

From Jean-Marc Falcoz, Jan 11 2009: (Start)

This is correct.

There can't be any more terms, because they must necessarily be of the form

23737373733737... but the substring 237 is composite

or 273737373... but 273 is composite

or 5373737373... but 537 is composite

or 5737373737... but 573 is composite

or 37373737373... but 3737 is composite

or 7373737373... but 737 is composite

No other form is possible, otherwise, if the digit 2 or 5 is anywhere inside or at the end of the number, one substring-number is even or divisible by 5, and furthermore, there can't be twin digits, because one substring-number would then be divisible by 11.

Obviously, the digits 0, 1, 4, 6, 8, 9 can't appear anywhere in a term of the sequence. (End)

Subsequence of A024770 (right-truncatable primes), A068669 (noncomposite numbers in which all substrings are noncomposite). Supersequence of A202263 (primes in which all substrings and reversal substrings are primes). - Jaroslav Krizek, Jan 28 2012.

From Hieronymus Fischer, Apr 20 2012: (Start)

A more general definition is "Numbers such that all substrings of length <= 3 are primes". Proof: For numbers < 1000 this is plainly true. Suppose that there are such n >= 1000. We recognize that n must contain the string 373, as this is the only valid prime substring with the length 3. It follows, that there are substrings x37 or 73x, with any digit x. Evidently, neither x37 nor 73x are valid prime substrings, independent from the digit x. Thus, there is no number >= 1000 such that all substrings of length <= 3 are primes.

Also, numbers such that all substrings of length <= 2 are primes and the number of prime substrings of length = 3 is greater than m-3 for n <= 37373 and is greater than min(m-4,floor((m-1)/2) else; where m=floor(log_10(a(n)))+1 = number of digits. (End)

The divisors of 24 greater than 1 are the only positive integers n with the property m^2 == 1 (mod n) for all integer m coprime to n. - Antonio G. Astudillo (afg_astudillo(AT)hotmail.com), Jun 10 2001

Numbers n for which all Dirichlet characters are real. - Benoit Cloitre, Apr 21 2002

These are the numbers n that are divisible by all numbers less than or equal to the square root of n. - Tanya Khovanova, Dec 10 2006 [For a proof, see the Tauvel paper in references. - Bernard Schott, Dec 20 2012]

Also, numbers n such that A160812(n) = 0. - Omar E. Pol, Jun 19 2009

It appears that these are the only positive integers n such that A160812(n) = 0. - Omar E. Pol, Nov 17 2009

24 is a highly composite number: A002182(6)=24. - Reinhard Zumkeller, Jun 21 2010

Chebolu points out that these are exactly the numbers for which the multiplication table of the integers mod n have 1s only on their diagonal, i.e., ab == 1 (mod n) implies a = b (mod n). - Charles R Greathouse IV, Jul 06 2011

It appears that 3, 4, 6, 8, 12, 24 (the divisors >= 3 of 24) are also the only numbers n whose proper non-divisors k are prime numbers if k = d-1 and d divides n. - Omar E. Pol, Sep 23 2011

About the last Pol's comment: I have searched to 10^7 and have found no other terms. - Robert G. Wilson v, Sep 23 2011

Sum_{i=1..8} A000005(a(i))^3 = (Sum_{i=1..8} A000005(a(i)))^2, see Kordemsky in References and Barbeau et al. in Links section. - Bruno Berselli, Dec 29 2014

Could also be called Gauss numbers, since he discovered them. Heegner proved list is complete. - Artur Jasinski, Mar 21 2003

Numbers n such that Q(sqrt(-n)) has unique factorization into primes.

These are the squarefree values of n for which if some positive integer N can be written in the form (a/2)^2+n*(b/2)^2 for integers a and b, then every prime factor P of N which occurs to an odd power can also be written in the form (c/2)^2+n*(d/2)^2 for integers c and d. - V. Raman, Sep 17 2012, May 01 2013

Cases n = 1 and n = 2 correspond to the rings Z[i] (Gaussian integers) and Z[sqrt(-2)] = numbers of the form a + b*sqrt(-2), where a and b are integers. Other cases, satisfying a(n) == 3 (mod 4), correspond to the rings of numbers of the form (a/2) + (b/2)*sqrt(-a(n)), for integers a and b of the same parity. All these rings admit unique factorization. - V. Raman, Sep 17 2012, corrected by Eric M. Schmidt, Feb 17 2013

The Heegner numbers greater than 3 can also be found using the Kronecker symbol, as follows: A number k > 3 is a Heegner number if and only if s = Sum_{j = 1..k} j * (j|k) is prime, which happens to be negative, where (x|y) is the Kronecker symbol. Also note for these results s = -k. But if s = -k is used as the selection condition (instead of primality), then the cubes of {7, 11, 19, 43, 67, 163} are also selected, followed by these same numbers to 9th power (and presumably followed by the 27th or 81st power). - Richard R. Forberg, Jul 18 2016

Theorem: The ring of integers of the imaginary quadratic field Q(sqrt(-n)) is Euclidean iff n = 1, 2, 3, 7 and 11. (Otherwise, the ring of integers of the imaginary quadratic field Q(sqrt(-n)) is principal iff n is a term of this sequence) [Link Stark-Heegner theorem]. - Bernard Schott, Feb 07 2020

Named after the German high school teacher and radio engineer Kurt Heegner (1893-1965). - Amiram Eldar, Jun 15 2021

Primes in which repeatedly deleting the least significant digit gives a prime at every step until a single-digit prime remains. The sequence ends at a(83) = 73939133 = A023107(10).

The subsequence which consists of the following "chain" of consecutive right truncatable primes: 73939133, 7393913, 739391, 73939, 7393, 739, 73, 7 yields the largest sum, compared with other chains formed from subsets of this sequence: 73939133 + 7393913 + 739391 + 73939 + 7393 + 739 + 73 + 7 = 82154588. - Alexander R. Povolotsky, Jan 22 2008

Can also be seen as a table whose n-th row lists the n-digit terms; row lengths (0 for n >= 9) are given by A050986. The sequence can be constructed starting with the single-digit primes and appending, for each p in the list, the primes within 10*p and 10(p+1), formed by appending a digit to p. - M. F. Hasler, Nov 07 2018

There are many equivalent definitions of these numbers. Based on Cox, Theorem 3.22 and Proposition 3.24 and a comment by Eric Rains (rains(AT)caltech.edu), we can say that a positive number n belongs to this sequence if and only if any of the following equivalent statements is true:

(1) Let m > 1 be an odd number relatively prime to n which can be written in the form x^2 + n*y^2 with x, y relatively prime. If the equation m = x^2 + n*y^2 has only one solution with x, y >= 0, then m is a prime number. [Euler]

(2) Every genus of quadratic forms of discriminant -4n consists of a single class. [Gauss]

(3) If a*x^2 + b*x*y + c*y^2 is a reduced quadratic form of discriminant -4n, then either b=0, a=b or a = c. [Cox]

(4) Two quadratic forms of discriminant -4n are equivalent if and only if they are properly equivalent. [Cox]

(5) The class group C(-4n) is isomorphic to (Z/2Z)^m for some integer m. [Cox]

(6) n is not of the form ab+ac+bc with 0 < a < b < c. (See proof in link below.) [Rains]

It is conjectured that the list given here is complete. Chowla showed that the list is finite and Weinberger showed that there is at most one further term.

If an additional term exists it is > 100000000. - Jud McCranie, Jun 27 2005

The terms shown are the union of {1,2,3,4,7}, A033266, A033267, A033268 and A033269 (corresponding to class numbers 1, 2, 4, 8 and 16 respectively).

Note that for n in this sequence, n+1 is either a prime, twice a prime, the square of a prime, 8 or 16. - T. D. Noe, Apr 08 2004. [This is a general theorem that is not hard to prove using genus theory. The "32" in the original comment was an error. - Tom Hagedorn (hagedorn(AT)tcnj.edu), Dec 29 2008]

Also numbers n such that for all primes p such that p is a quadratic residue (mod 4*n) and p-n is a quadratic residue (mod 4*n), p can be uniquely written into the form as x^2+n*y^2. - V. Raman, Nov 25 2013