OFFSET
1,1
COMMENTS
Even numbers k such that the symmetric representation of sigma(k) has an even number of parts.
For the definition of middle divisors, see A067742.
For more information about the symmetric representation of sigma(k) see A237593.
Let p be a prime > 5. Then a(n) is a number of the form m*p where m is an even number < sqrt(p). - David A. Corneth, Sep 28 2018
First differs from A244894 at a(51) = 230. - R. J. Mathar, Oct 04 2018
Is this twice A101550? - Omar E. Pol, Oct 04 2018
This sequence is not twice A101550: first differs at a(57) = 250 != 254 = 2*A101550(57). - Michael S. Branicky, Oct 14 2021
LINKS
Michael S. Branicky, Table of n, a(n) for n = 1..10000
EXAMPLE
10 is in the sequence because it's an even number and the symmetric representation of sigma(10) = 18 has an even number of parts as shown below:
.
. _ _ _ _ _ _ 9
. |_ _ _ _ _ |
. | |_
. |_ _|_
. | |_ _ 9
. |_ _ |
. | |
. | |
. | |
. | |
. |_|
.
PROG
(Python)
from sympy import divisors
def ok(n):
if n < 2 or n%2 == 1: return False
return not any(n//2 <= d*d < 2*n for d in divisors(n, generator=True))
print(list(filter(ok, range(285)))) # Michael S. Branicky, Oct 14 2021
CROSSREFS
KEYWORD
nonn
AUTHOR
Omar E. Pol, Sep 28 2018
STATUS
approved