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A111390
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a(1)=1. a(n) = smallest positive integer not occurring earlier in the sequence such that |d(a(n))-d(a(n-1))| = 1, where d(n) is the number of positive divisors of n.
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5
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1, 2, 4, 3, 9, 5, 25, 6, 16, 8, 49, 7, 121, 10, 81, 12, 64, 18, 625, 14, 169, 11, 289, 13, 361, 15, 529, 17, 841, 19, 961, 21, 1369, 22, 1681, 23, 1849, 26, 2209, 27, 2401, 20, 729, 24, 36, 30, 100, 40, 196, 42, 225, 48, 256, 54, 441, 56, 484, 66, 676, 70, 1089, 78, 1156, 80
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| Sequence is a permutation of the positive integers.
Terms a(65) to a(76) are 1024, 60, 4096, 72, 59049, 84, 531441, 90, 9765625, 96, 244140625, 108. - Klaus Brockhaus, Nov 13 2005
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EXAMPLE
| Among positive integers not among the first 4 terms of the sequence, a(5) = 9 is the lowest such that |d(a(5))-d(a(4))| = |d(9)-d(3)| = |3-2| is 1.
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CROSSREFS
| Cf. A114107 (inverse), A114108 (number of divisors), A114109 (fixed points), A114110 (records), A114111 (where records occur).
Sequence in context: A063379 A000463 A137442 * A129596 A157182 A060837
Adjacent sequences: A111387 A111388 A111389 * A111391 A111392 A111393
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KEYWORD
| nonn
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AUTHOR
| Leroy Quet Nov 10 2005
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EXTENSIONS
| More terms from Klaus Brockhaus (klaus-brockhaus(AT)t-online.de), Nov 11 2005
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