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A074486
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Decimal encoding of topologies generated by classes of sets: map {}, a, b, c, d, ... to 1, 2, 4, 16, 256, ...i.e. ( 2^0, 2^1, 2^2, 2^4, 2^8, ...).
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1
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1, 3, 9, 11, 15, 129, 131, 137, 139, 143, 153, 171, 175, 255
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,2
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COMMENTS
| The sequence encodes unlabeled topologies as described in A000798. For example, a(3)= 1+2+8 = 11 since {}, a,ab is the least decimal encoding. 1+4+8 = 13 since {}, b,ab.But is topologically equivalent; so is not in a(n). The number of equivalent cases corresponding to a(n) begins 1; 1,1,2; 1,1,3,3,6,3,3,3,6; ... and is counted by A001928 (labeled topologies).
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EXAMPLE
| a(11) = 171 because we can map {}, a, ab, ac, abc to 1 + 2 + 8 + 32 + 128
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CROSSREFS
| Cf. A000798.
Sequence in context: A190226 A059326 A028312 * A131861 A131859 A096187
Adjacent sequences: A074483 A074484 A074485 * A074487 A074488 A074489
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KEYWORD
| more,nonn
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AUTHOR
| Alford Arnold (Alford1940(AT)aol.com), Sep 26 2002
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