%I
%S 1,1,2,3,2,3,3,5,4,4,3,8,4,5,6,9,4,8,5,10,8,7,5,15,7,8,9,13,6,14,7,15,
%T 10,10,10,20,8,11,12,20,8,17,9,17,16,13,9,28,11,17,14,20,10,22,14,25,
%U 16,16,11,34,12,17,20,27,16,26,13,24,18,24,13,40
%N Number of inequivalent sublattices of index n in hexagonal lattice, where two sublattices are considered equivalent if they are related by any rotation or reflection.
%C If we count sublattices as equivalent only if they are related by a rotation, we get A054384 instead of this sequence. If we only allow rotations and reflections that preserve the parent (hexagonal) lattice, we get A003051; the first discrepancy is at n = 42 (see illustration), the second is at n = 49. If both restrictions are applied, i.e., only rotations preserving the parent lattice are allowed, we get A145394. The analog for square lattice is A054346.
%C Although A003051 has its counterpart A003050 which counts primitive sublattices only, this sequence has no such counterpart sequence because a primitive sublattice can turn to a nonprimitive one via a nonparentlatticepreserving rotation, so the straightforward definition of primitiveness does not work in this case.
%H Andrey Zabolotskiy, <a href="/A300651/b300651.txt">Table of n, a(n) for n = 1..1000</a>
%H Andrey Zabolotskiy, <a href="/A145394/a145394.pdf">Sublattices of the hexagonal lattice</a> (illustrations for n = 1..7, 14)
%H <a href="/index/Su#sublatts">Index entries for sequences related to sublattices</a>
%H <a href="/index/Aa#A2">Index entries for sequences related to A2 = hexagonal = triangular lattice</a>
%Y Cf. A003050, A003051, A054384, A145394, A054346.
%K nonn
%O 1,3
%A _Andrey Zabolotskiy_, Mar 10 2018
