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A248151 Numbers n such that the sum of the divisors of n is not divisible by 4. 5
1, 2, 4, 5, 8, 9, 10, 13, 16, 17, 18, 20, 25, 26, 29, 32, 34, 36, 37, 40, 41, 45, 49, 50, 52, 53, 58, 61, 64, 68, 72, 73, 74, 80, 81, 82, 89, 90, 97, 98, 100, 101, 104, 106, 109, 113, 116, 117, 121, 122, 128, 136, 137, 144, 146, 148, 149, 153, 157, 160, 162, 164, 169, 173, 178, 180 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

The complement of A248150; contains in particular A028982, the squares and twice the squares, for which sigma is odd.

The terms cannot have more than one odd prime factor to an odd power. Therefore this set has asymptotic density zero. The sequence grows faster than .75 n log(n). In particular: a(10) = 17, a(100) = 298, a(10^3) = 4724, a(10^4) = 66533, a(10^5) = 872434, a(10^6) = 10819205. - M. F. Hasler, Apr 26 2017

LINKS

M. F. Hasler, Table of n, a(n) for n = 1..1000

PROG

(PARI) for(n=1, 999, sigma(n)%4&&print1(n", "))

CROSSREFS

Cf. A000203 (sum of divisors), A028982, A028983, A248150.

Sequence in context: A269176 A091072 A001481 * A245226 A034026 A125022

Adjacent sequences:  A248148 A248149 A248150 * A248152 A248153 A248154

KEYWORD

nonn

AUTHOR

M. F. Hasler, Oct 02 2014

STATUS

approved

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Last modified June 6 01:16 EDT 2020. Contains 334858 sequences. (Running on oeis4.)