login
A334525
Number of ordered triples (w,x,y) with all terms in {-n,...,0,...,n} and w^2 + 7xy = 0.
1
1, 5, 9, 13, 17, 21, 25, 37, 41, 45, 49, 53, 57, 61, 81, 85, 89, 93, 97, 101, 105, 125, 129, 133, 137, 141, 145, 149, 185, 189, 193, 197, 201, 205, 209, 229, 233, 237, 241, 245, 249, 253, 297, 301, 305, 309, 313, 317, 321, 333, 337, 341, 345, 349, 353, 357, 393, 397
OFFSET
0,2
COMMENTS
If n is squarefree and not divisible by 7, a(n) = a(n-1)+4. - Robert Israel, Jul 01 2020
LINKS
MAPLE
df:= proc(n) local t, s, m0, m;
if n mod 7 = 0 then
m:= n/7;
t:= 4*nops(select(s -> s < n and s > m, numtheory:-divisors(7*m^2)))
else t:= 0
fi;
m0:= mul(`if`(s[1]=7, s[1]^ceil((s[2]-1)/2),
s[1]^ceil(s[2]/2)), s=ifactors(n)[2]);
t + 4 + 8*floor(n/m0/7);
end proc:
df(0):= 1:
ListTools:-PartialSums(map(df, [$0..100])); # Robert Israel, Jul 01 2020
CROSSREFS
Cf. A211423.
Sequence in context: A314674 A314675 A314676 * A314677 A190951 A314678
KEYWORD
nonn
AUTHOR
Brandon Crofts, Jun 15 2020
STATUS
approved