A291338 Number of n X n X n triangular binary arrays symmetric under rotations of 120 degrees.

2, 2, 4, 16, 32, 128, 1024, 4096, 32768, 524288, 4194304, 67108864, 2147483648, 34359738368, 1099511627776, 70368744177664, 2251799813685248, 144115188075855872, 18446744073709551616, 1180591620717411303424, 151115727451828646838272

Offset

1,1

Links

Table of n, a(n) for n=1..21.

Formula

a(n) = 2^ceiling(n(n+1)/6). - Giovanni Resta, Aug 23 2017

Example

All solutions for n = 3:
    0      0      1      1
   0 0    1 1    0 0    1 1
  0 0 0  0 1 0  1 0 1  1 1 1

Mathematica

Table[2^Ceiling[n (n + 1)/6], {n, 20}] (* Giovanni Resta, Aug 23 2017 *)

Prog

(PARI) a(n) = 2^ceil(n*(n+1)/6) \\ after Giovanni Resta's formula

Crossrefs

Cf. A166197, A166198, A166199, A166200, A166201, A166202, A166203, A166204, A166205, A166206, A166207, A166208, A166209, A166210, A166211, A166212, A166213, A166214, A166215, A166216, A166217, A166218, A166219, A166220, A166221, A166222, A166223, A166224.

Sequence in context: A153997 A154000 A154003 * A326481 A009542 A244108

Adjacent sequences: A291335 A291336 A291337 * A291339 A291340 A291341

Keyword

nonn,easy

Author

Felix Fröhlich, Aug 22 2017

Extensions

More terms from Giovanni Resta, Aug 23 2017

Status

approved