

A003136


Loeschian numbers: numbers of the form x^2 + xy + y^2; norms of vectors in A2 lattice.
(Formerly M2336)


53



0, 1, 3, 4, 7, 9, 12, 13, 16, 19, 21, 25, 27, 28, 31, 36, 37, 39, 43, 48, 49, 52, 57, 61, 63, 64, 67, 73, 75, 76, 79, 81, 84, 91, 93, 97, 100, 103, 108, 109, 111, 112, 117, 121, 124, 127, 129, 133, 139, 144, 147, 148, 151, 156, 157, 163, 169, 171, 172, 175, 181, 183, 189, 192
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OFFSET

1,3


COMMENTS

Equally, numbers of form x^2  xy + y^2.  Ray Chandler, Jan 27 2009
Also, numbers of form X^2+3Y^2 (X=y+x/2, Y=x/2). Cf. A092572 Numbers of the form x^2+3y^2 where x and y are positive integers.  Zak Seidov, Jan 20 2009
Equivalently, numbers n such that the coefficient of x^n in Theta3(x)*Theta3(x^3) is nonzero.  Joerg Arndt, Jan 16 2011.
Equivalently, numbers n such that the coefficient of x^n in a(x) (resp. b(x)) is nonzero where a(), b() are cubic AGM functions.  Michael Somos, Jan 16 2011
Relative areas of equilateral triangles whose vertices are on a triangular lattice.  Anton Sherwood (bronto(AT)pobox.com), Apr 05 2001
2 appended to a(n) (for positive n) corresponds to capsomere count in viral architectural structures (cf. A071336).  Lekraj Beedassy, Apr 14 2006
The triangle in A132111 gives the enumeration: n^2 + k*n + k^2, 0<=k<=n.
The number of coat proteins at each corner of a triangular face of a virus shell.  Parthasarathy Nambi, Sep 04 2007
A088534(a(n)) > 0.  Reinhard Zumkeller, Oct 30 2011
Numbers of the form (x^2 + y^2 + (x + y)^2)/2. If we let z =  x  y, then all the solutions to x^2 + y^2 + z^2 = k with x + y + z = 0 are k = 2a(n) for any n.  Jon Perry, Dec 16 2012
Sequence of divisors of the hexagonal lattice, except zero (where it is said that an integer n divides a lattice if there exists a sublattice of index n; example: 3 divides the hexagonal lattice).  JeanChristophe Hervé, May 01 2013
Numbers of the form  (x*y + y*z + x*z) with x + y + z = 0. Numbers of the form x^2 + y^2 + z^2  (x*y + y*z + x*z) = (x  y)*(x  z) + (y  x) * (y  z) + (z  x) * (z  y).  Michael Somos, Jun 26 2013


REFERENCES

J. H. Conway and N. J. A. Sloane, "Sphere Packings, Lattices and Groups", SpringerVerlag, p. 111.
J. U. Marshall, The Loeschian numbers as a problem in number theory, Geographical Analysis, 7 (1975), 421426.
Ivars Peterson, "The Jungles of Randomness: A Mathematical Safari", John Wiley and Sons, (1998) pp. 53.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).


LINKS

T. D. Noe, Table of n, a(n) for n = 1..10000
M. Bernstein, N. J. A. Sloane and P. E. Wright, On Sublattices of the Hexagonal Lattice, Discrete Math. 170 (1997) 2939 (Abstract, pdf, ps).
J. H. Conway, E. M. Rains and N. J. A. Sloane, On the existence of similar sublattices, Canad. J. Math. 51 (1999), 13001306 (Abstract, pdf, ps).
August Lösch, Economics of Location (1954), see pp. 117f.
U. P. Nair, Elementary results on the binary quadratic form a^2+ab+b^2
Index entries for sequences related to A2 = hexagonal = triangular lattice
Index entries for "core" sequences


FORMULA

Either n=0 or else in the prime factorization of n all primes of the form 3a+2 must occur to even powers only (there is no restriction of primes congruent to 0 or 1 mod 3).
If n is in the sequence, then n^k is in the sequence (but the converse is not true). n is in the sequence iff n^(2k+1) is in the sequence.  Ray Chandler, Feb 03 2009
If m and n are in the sequence, so is m*n.  Jon Perry, Dec 18 2012


MAPLE

readlib(ifactors): for n from 2 to 200 do m := ifactors(n)[2]: flag := 1: for i from 1 to nops(m) do if m[i, 1] mod 3 = 2 and m[i, 2] mod 2 = 1 then flag := 0; break fi: od: if flag=1 then printf(`%d, `, n) fi: od: # James A. Sellers, Dec 07 2000


MATHEMATICA

ok[n_] := Resolve[Exists[{x, y}, Reduce[n == x^2 + x*y + y^2, {x, y}, Integers]]]; Select[Range[0, 192], ok] (* JeanFrançois Alcover, Apr 18 2011 *)
nn = 14; Select[Union[Flatten[Table[x^2 + x*y + y^2, {x, 0, nn}, {y, 0, x}]]], # <= nn^2 &] (* T. D. Noe, Apr 18 2011 *)


PROG

(Haskell)
import Data.Set (singleton, union, fromList, deleteFindMin)
a003136 n = a003136_list !! (n1)
a003136_list = f 0 $ singleton 0 where
f x s  m < x ^ 2 = m : f x s'
 otherwise = m : f x'
(union s' $ fromList $ map (\y > x'^2+(x'+y)*y) [0..x'])
where x' = x + 1
(m, s') = deleteFindMin s
 Reinhard Zumkeller, Oct 30 2011
(PARI) isA003136(n)={local(fac, flag); if(n==0, 1, fac=factor(n); flag=1; for(i=1, matsize(fac)[1], if(Mod(fac[i, 1], 3)==2 && Mod(fac[i, 2], 2)==1, flag=0)); flag)}
(PARI) is(n)=#bnfisintnorm(bnfinit(z^2+z+1), n) \\ Ralf Stephan, Oct 18 2013


CROSSREFS

Subsequence of A032766.
Cf. A004611, A034017, A045897, A060428.
See A092572 for numbers of form x^2 + 3 y^2 with positive x,y.
Cf. A034020 (complement), A007645 (primes); partitions: A198726, A198727; subsequences: A198772, A118886, A198773, A198774, A198775.
Sequence in context: A120451 A060428 A035238 * A034022 A198772 A185256
Adjacent sequences: A003133 A003134 A003135 * A003137 A003138 A003139


KEYWORD

core,easy,nonn,nice


AUTHOR

N. J. A. Sloane


STATUS

approved



