OFFSET
1,1
COMMENTS
Deficient numbers a(n) for which a(n)-1 and a(n)+1 are not deficient. In most instances, both a(n)-1 and a(n)+1 will be abundant numbers; in a few instances, one will be abundant while the other one is perfect.
LINKS
Harvey P. Dale, Table of n, a(n) for n = 1..1000 (First 413 terms from Timothy L. Tiffin)
EXAMPLE
19 is between two abundant numbers (18 and 20), while 29 is between a perfect number (28) and an abundant number (30).
MATHEMATICA
Select[Range@ 750, Boole@ Map[DivisorSigma[1, #] < 2 # &, # + {-1, 0, 1}] == {0, 1, 0} &] (* Michael De Vlieger, Aug 26 2016 *)
SequencePosition[Table[If[DivisorSigma[1, n]<2n, 1, 0], {n, 800}], {0, 1, 0}][[All, 1]]+1 (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Sep 21 2017 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Timothy L. Tiffin, Jul 08 2016
STATUS
approved