OFFSET
0,2
LINKS
Wikipedia, Vesica piscis
FORMULA
a(n) = 1 - 3*n - 2*(n-1)*m(n) + 4 * Sum_{k=0..m(n)} floor(sqrt(n^2-k^2)) where m(n) = floor(n*sqrt(3)/2). - Franz Vrabec, Oct 02 2016
a(n)/n^2 tends to A093731 as n tends to infinity. - Rémy Sigrist, Mar 07 2021
PROG
(PARI) a(n) = my(m = floor(n*sqrt(3)/2)); 1 - 3*n - 2*(n-1)*m + 4*sum(k=0, m, sqrtint(n^2-k^2)); \\ Michel Marcus, Mar 07 2021
CROSSREFS
KEYWORD
nonn
AUTHOR
Christina Steffan, Sep 05 2016
EXTENSIONS
More terms from Franz Vrabec, Oct 02 2016
STATUS
approved