

A088828


Nonsquare positive odd numbers.


6



3, 5, 7, 11, 13, 15, 17, 19, 21, 23, 27, 29, 31, 33, 35, 37, 39, 41, 43, 45, 47, 51, 53, 55, 57, 59, 61, 63, 65, 67, 69, 71, 73, 75, 77, 79, 83, 85, 87, 89, 91, 93, 95, 97, 99, 101, 103, 105, 107, 109, 111, 113, 115, 117, 119, 123, 125, 127, 129, 131, 133, 135, 137, 139
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OFFSET

1,1


COMMENTS

Odd numbers with even abundance: primes and some composites too.
Odd numbers with odd abundance are in A016754. Even numbers with odd abundance are in A088827. Even numbers with even abundance are in A088829.
Or, odd numbers without the squares.  Gerald Hillier, Apr 12 2009


LINKS

Table of n, a(n) for n=1..64.


FORMULA

Let s = round(sqrt(2*n1)); then a(n) = 2*n + s  ((s+1) mod 2).  Gerald Hillier, Apr 15 2009
A005408 SETMINUS A016754.  R. J. Mathar, Jun 16 2018


EXAMPLE

n = p prime, abundance = 1  p = even and negative;
n = 21, sigma = 1 + 3 + 7 + 21 = 32, abundance = 32  42 = 20.


MATHEMATICA

Do[s=DivisorSigma[1, n]2*n; If[ !OddQ[s]&&OddQ[n], Print[{n, s}]], {n, 1, 1000}]
Select[Range[1, 500, 2], EvenQ[DivisorSigma[1, #]  2 #] &] (* Vladimir Joseph Stephan Orlovsky, Apr 15 2011 *)


PROG

(MAGMA) [ n: n in [1..140 by 2]  IsEven(SumOfDivisors(n)2*n) ]; // Klaus Brockhaus, Apr 15 2009
(PARI) isok(n) = (n>0) && (n % 2) && ! issquare(n); \\ Michel Marcus, Aug 28 2013


CROSSREFS

Cf. A016754, A088827, A088829, A157502.
Sequence in context: A100933 A320056 A175679 * A182318 A247424 A305635
Adjacent sequences: A088825 A088826 A088827 * A088829 A088830 A088831


KEYWORD

nonn,easy


AUTHOR

Labos Elemer, Oct 28 2003


EXTENSIONS

Entry revised by N. J. A. Sloane, Jan 31 2014 at the suggestion of Omar E. Pol


STATUS

approved



