

A057750


Number of nonfactorable subsets of size >= 2 of a 1 X n uniform grid.


2



0, 1, 4, 10, 23, 49, 100, 202, 413, 839, 1713, 3493, 7130, 14535, 29617, 60158, 122077, 247132, 499409, 1007440, 2029801, 4083888, 8208828, 16484742, 33081031, 66342016, 132979769, 266433643, 533645668, 1068542949, 2139103450, 4281451287, 8568182883
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OFFSET

1,3


COMMENTS

A set is factorable if it is the union of at least two disjoint translated copies of a subset of at least two elements. E.g. the subset *..*.**..***.*.* of the 1x16 grid (where * denotes gridpoints in the selected subset and . denotes the remaining unselected gridpoints) is factorable into 3 copies of the 3element subset *..*.*, as shown by displaying the factors by 1..1.12..232.3.3, where the numerals denote the elements of a particular translated copy.


LINKS



FORMULA



EXAMPLE

The factorable subsets of (......) are (1122..), (11.22.), (.1122.), (1.12.2), (11..22), (.11.22), (..1122) and (111222) and there are seven subsets with fewer than 2 elements, so a(6)=2^687=49.


CROSSREFS



KEYWORD

nonn


AUTHOR



EXTENSIONS



STATUS

approved



