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A055927
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Numbers k such that k+1 has one more divisor than k.
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3
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1, 3, 9, 15, 25, 63, 121, 195, 255, 361, 483, 729, 841, 1443, 3363, 3481, 3721, 5041, 6241, 10201, 15625, 17161, 18224, 19321, 24963, 31683, 32761, 39601, 58564, 59049, 65535, 73441, 88208, 110889, 121801, 143641, 145923, 149769, 167281
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| Numbers k such that d(k+1)-d(k)=1, where d() is A000005, the number of divisors.
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EXAMPLE
| a(4) = 15, as 15 has 4 and 16 has 5 divisors. a(6) = 63, as 63 and 64 have 6 and 7 divisors respectively.
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MATHEMATICA
| Select[ Range[ 200000], DivisorSigma[0, # ] + 1 == DivisorSigma[0, # + 1] &]
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PROG
| (Pari) for(n=1, 1000, if(numdiv(n+1)-numdiv(n)==1, print1(n, ", "))); /* Joerg Arndt, Apr 09 2011 */
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CROSSREFS
| Numbers where repetition occurs in A049820.
Cf. A000005, A049820, A075041, A045983, A006073, A075044.
Sequence in context: A099989 A085046 A138495 * A087031 A089632 A082897
Adjacent sequences: A055924 A055925 A055926 * A055928 A055929 A055930
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KEYWORD
| nonn
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AUTHOR
| Labos E. (labos(AT)ana.sote.hu), Jul 21 2000
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EXTENSIONS
| More terms from David W. Wilson (davidwwilson(AT)comcast.net), Sep 06 2000, who remarks that every element is of form n^2 or n^2-1.
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