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A011955
Number of Barlow packings with group R3(bar)m(O) that repeat after 6n layers.
2
1, 2, 4, 9, 19, 40, 80, 165, 330, 672, 1344, 2709, 5418, 10878, 21760, 43605, 87211, 174592, 349180, 698707, 1397418, 2795520, 5591040, 11183436, 22366890, 44736512, 89473020, 178951509, 357903000, 715816960, 1431633920, 2863289683, 5726579370, 11453202383, 22906404864, 45812897109, 91625794218
OFFSET
2,2
LINKS
MAPLE
# eq (6) in Iglesias Z Krist. 221 (2006)
b := proc(p, q)
local d;
a := 0 ;
for d from 1 to min(p, q) do
if modp(p, d)=0 and modp(q, d)=0 then
ph := floor(p/2/d) ;
qh := floor(q/2/d) ;
a := a+numtheory[mobius](d)*binomial(ph+qh, ph) ;
end if ;
end do:
a ;
end proc:
# eq (17) in Iglesias Z Krist. 221 (2006)
bt := proc(p, q)
if type(p+q, 'odd') then
b(p, q) ;
else
0;
end if;
end proc:
# corrected eq (15) in Iglesias Z Krist. 221 (2006), d|(p/2) and d|(q/2)
bbtemp := proc(p, q)
local d, ph, qh;
a := 0 ;
for d from 1 to min(p, q) do
if modp(p, 2*d)=0 and modp(q, 2*d)=0 then
ph := p/2/d ;
qh := q/2/d ;
a := a+numtheory[mobius](d)*binomial(ph+qh, ph) ;
end if ;
end do:
a ;
end proc:
# eq (16) in Iglesias Z Krist. 221 (2006)
bb := proc(p, q)
if type(p, 'even') and type(q, 'even') then
( bbtemp(p, q)-bt(p/2, q/2) )/2 ;
else
0 ;
end if;
end proc:
tt := proc(p, q)
if type(p+q, 'odd') then
0 ;
else
b(p, q)-bb(p, q);
end if;
end proc:
# eq (29) in Iglesias
A011955 := proc(n)
local a, p, q, P ;
P := 2*n ;
a :=0 ;
for q from 0 to P do
p := P-q ;
if modp(p-q, 3) <> 0 and p < q then
a := a+tt(p, q) ;
end if;
end do:
a ;
end proc:
seq(A011955(n), n=2..40) ; # R. J. Mathar, Apr 15 2024
CROSSREFS
Sequence in context: A099568 A018001 A018099 * A084172 A018100 A052908
KEYWORD
nonn,easy
AUTHOR
STATUS
approved