A245810 Genus of the complete intersection of two hypersurfaces in P^3.

0, 1, 3, 4, 6, 9, 10, 15, 16, 19, 21, 25, 28, 31, 33, 36, 45, 46, 49, 51, 55, 64, 66, 73, 76, 78, 81, 85, 91, 99, 100, 105, 106, 109, 120, 121, 129, 136, 141, 144, 145, 153, 163, 166, 169, 171, 181, 190, 196, 199, 201, 210, 225, 226, 231, 235, 241, 243

Offset

1,3

Comments

Integers that can be written as d*e*(d+e-4)/2+1 for positive integers d and e.

Links

Jean-François Alcover, Table of n, a(n) for n = 1..10000

Maple

filter:= proc(n)
local m, S, Q;
m:= 2*(n-1);
S:= numtheory:-divisors(m);
Q:= map(d ->(4*d-m+sqrt(4*d^3 + 16*d^2 - 8*d*m + m^2))/(2*d), S);
evalb(select(type, Q, posint) <> {})
end proc:
filter(0):= true:
filter(1):= true:
select(filter, [$0..1000]); # Robert Israel, Aug 24 2014

Mathematica

Module[{gmax = 300, g}, Reap[Do[g = d e (d + e - 4)/2 + 1; If[IntegerQ[g], Sow[g]], {d, Ceiling[(3 + Sqrt[1 + 8 gmax])/2]}, {e, d}]][[2, 1]] // Select[#, # <= gmax &]& // Union] (* Jean-François Alcover, Sep 12 2020 *)

Crossrefs

Sequence in context: A368136 A130904 A034706 * A054686 A005214 A268110

Adjacent sequences: A245807 A245808 A245809 * A245811 A245812 A245813

Keyword

nonn

Author

Richard Shadrach, Aug 22 2014

Extensions

b-file corrected by Jean-François Alcover, Sep 12 2020

Status

approved