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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
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
PROG
(PARI) for(n=1, 999, sigma(n)%4&&print1(n", "))
CROSSREFS
Cf. A000203 (sum of divisors), A028982, A028983, A248150.
Subsequence of A285801.
Sequence in context: A269176 A091072 A001481 * A245226 A034026 A125022
KEYWORD
nonn
AUTHOR
M. F. Hasler, Oct 02 2014
STATUS
approved