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A074918 Highly imperfect numbers: n sets a record for the value of abs(sigma(n)-2*n) (absolute value of A033879). 5
1, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 120, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 180, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 240, 269, 271, 277, 281, 283, 293, 307 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

A perfect number n is defined by sigma(n) = 2n, so the value of i(n) = |sigma(n)-2n| measures the degree of perfection of n. The larger i(n) is, the more "imperfect" n is. I call the numbers n such that i(k) < i(n) for all k < n "highly-imperfect numbers".

RECORDS transform of |A033879|.

Initial terms are odd primes but then even numbers appear.

The last odd term is a(79) = 719. (Proof: sigma(27720n) >= 11080n, and so sigma(27720n) >= 4 * 27720(n + 1) for n >= 8, so there is no odd member of this sequence between 27720 * 8 and 27720 * 9, between 27720 * 9 and 2770 * 10, etc.; the remaining terms are checked by computer.) [Charles R Greathouse IV, Apr 12 2010]

LINKS

R. J. Mathar and Donovan Johnson, Table of n, a(n) for n = 1..1000 (first 238 terms from R. J. Mathar)

Joseph L. Pe, Puzzle 241. Highly imperfect primes, and answers, on C. Rivera's primepuzzles.net. [From M. F. Hasler, May 31 2009]

N. J. A. Sloane, Transforms

MATHEMATICA

r = 0; l = {}; Do[ n = Abs[2 i - DivisorSigma[1, i]]; If[n > r, r = n; l = Append[l, i]], {i, 1, 10^4}]; l

CROSSREFS

Cf. A033879, A075728.

Sequence in context: A093893 A056912 A075763 * A175524 A073579 A006005

Adjacent sequences: A074915 A074916 A074917 * A074919 A074920 A074921

KEYWORD

easy,nice,nonn

AUTHOR

Joseph L. Pe, Oct 01 2002

STATUS

approved

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Last modified December 3 00:23 EST 2022. Contains 358510 sequences. (Running on oeis4.)