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A003136 Loeschian numbers: numbers of the form x^2 + xy + y^2; norms of vectors in A2 lattice.
(Formerly M2336)
56
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Equally, numbers of the form x^2 - xy + y^2. - Ray Chandler, Jan 27 2009.

Also, numbers of the form X^2+3Y^2 (X=y+x/2, Y=x/2), cf. A092572. - Zak Seidov, Jan 20 2009

Theorem (Schering, Delone, Watson): The only positive definite binary quadratic forms that represent the same numbers are x^2+xy+y^2 and x^2+3y^2 (up to scaling). - N. J. A. Sloane, Jun 22 2014

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). - Jean-Christophe 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", Springer-Verlag, p. 111.

B. N. Delone. Geometry of positive quadratic forms, addendum, Uspechi Mat. Sri, 4 (1938), 102-164.

Y. Kitaoka, On the relation between the positive-definite quadratic forms with the same representation numbers, Proc. Jap. Acad., 47 (1971), 439-441.

J. U. Marshall, The Loeschian numbers as a problem in number theory, Geographical Analysis, 7 (1975), 421-426.

Ivars Peterson, "The Jungles of Randomness: A Mathematical Safari", John Wiley and Sons, (1998) pp. 53.

M. Schering. Theoremes relatifs aux formes quadratiques qui representent les memes nombres, J. Math, pures el appl., 2 serie 4 (1859).

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

V. N. Timofeev. "On positive quadratic forms, representing the same numbers", Uspekhi Mat. Nauk, 18 (1963), 191-193.

Watson, G. L. Determination of a binary quadratic form by its values at integer points. Mathematika 26 (1979), no. 1, 72--75. MR0557128 (81e:10019).

Watson, G. L. Acknowledgement: "Determination of a binary quadratic form by its values at integer points'' [Mathematika 26 (1979), no. 1, 72-75; MR 81e:10019]. Mathematika 27 (1980), no. 2, 188 (1981). MR0610704 (82d:10037)

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) 29-39 (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), 1300-1306 (Abstract, pdf, ps).

William C. Jagy and Irving Kaplansky, Positive definite binary quadratic forms that represent the same primes [Cached copy]

August Lösch, Economics of Location (1954), see pp. 117f.

U. P. Nair, Elementary results on the binary quadratic form a^2+ab+b^2

N. J. A. Sloane et al., Binary Quadratic Forms and OEIS (Index to related sequences, programs, references)

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] (* Jean-François 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 !! (n-1)

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

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Last modified August 1 09:41 EDT 2014. Contains 245113 sequences.