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Search: id:A060798
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| A060798 |
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Difference between upper and lower central divisors of n is 1. |
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+0
1
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| 1, 2, 4, 6, 12, 20, 30, 42, 56, 72, 90, 110, 132, 156, 182, 210, 240, 272, 306, 342, 380, 420, 462, 506, 552, 600, 650, 702, 756, 812, 870, 930, 992, 1056, 1122, 1190, 1260, 1332, 1406, 1482, 1560, 1640, 1722, 1806, 1892, 1980, 2070, 2162, 2256, 2352, 2450
(list; graph; listen)
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OFFSET
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0,2
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LINKS
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Harry J. Smith, Table of n, a(n) for n=0,...,1000
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FORMULA
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Solutions to A033677(n)-A060775(n)=1, where n=k*(k+1) and at least one of k and k+1 is composite.
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EXAMPLE
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n = 4032 = 2.2.2.2.2.2.3.3.7 is here because its central [the 21-th and 22th] divisors are {63,64} with difference = 1. If n = 2^k(2^k-1) = 2^k*M or n = 2^k(2^k+1) = 2^k*F suitable M and F primes, then n is here (e.g. n = 272,992, etc.). This holds also for n = C*(C+1) products where C is composite and C+1 is prime,e.g. C = 2310.
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PROGRAM
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(PARI) { n=-1; for (m=1, 999000, d=divisors(m); if (m==1 || (d[1 + length(d)\2] - d[length(d)\2]) == 1, write("b060798.txt", n++, " ", m)); ) } [From Harry J. Smith (hjsmithh(AT)sbcglobal.net), Jul 13 2009]
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CROSSREFS
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Cf. A000196, A033677, A060775.
Sequence in context: A068911 A094769 A068018 this_sequence A134320 A107383 A078025
Adjacent sequences: A060795 A060796 A060797 this_sequence A060799 A060800 A060801
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KEYWORD
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nonn
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AUTHOR
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Labos E. (labos(AT)ana.sote.hu), Apr 27 2001
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