|
|
A281981
|
|
a(n) = 4*Sum_{i=1..n-1} Sum_{j=1..m} floor((j*i)/n) - (m-1)*m*(n-1) where m is floor(sqrt(n)).
|
|
0
|
|
|
0, 0, 0, 2, 0, 2, 0, 2, 4, 2, 0, 6, 0, 2, 4, 8, 0, 8, 0, 8, 4, 4, 0, 12, 8, 4, 4, 8, 0, 16, 0, 8, 4, 4, 8, 22, 0, 6, 8, 18, 0, 18, 0, 10, 16, 6, 0, 22, 12, 14, 8, 10, 0, 18, 8, 22, 8, 6, 0, 30, 0, 6, 20, 24, 8, 20, 0, 16, 8, 28, 0, 36, 0, 8, 16, 16, 12, 20, 0, 32
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
1,4
|
|
COMMENTS
|
Tsangaris proves that a(n)=0 iff n is prime (or 1) and a(n)>0 iff n is composite.
|
|
LINKS
|
|
|
PROG
|
(PARI) a(n) = if(iscomposite(n), my(m = sqrtint(n)); 4*sum(i=1, n-1, sum(j=1, m, (j*i)\n)) - (m-1)*m*(n-1), 0)
|
|
CROSSREFS
|
|
|
KEYWORD
|
nonn
|
|
AUTHOR
|
|
|
EXTENSIONS
|
|
|
STATUS
|
approved
|
|
|
|