A278481 Number of neighbors of the n-th term in a full isosceles triangle read by rows.

2, 4, 4, 4, 6, 4, 4, 6, 6, 4, 4, 6, 6, 6, 4, 4, 6, 6, 6, 6, 4, 4, 6, 6, 6, 6, 6, 4, 4, 6, 6, 6, 6, 6, 6, 4, 4, 6, 6, 6, 6, 6, 6, 6, 4, 4, 6, 6, 6, 6, 6, 6, 6, 6, 4, 4, 6, 6, 6, 6, 6, 6, 6, 6, 6, 4, 4, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 4, 4, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 4

Offset

1,1

Comments

Apart from the left border and the right border, the rest of the elements are 6's.

For the same idea but for a right triangle see A278480; for a square array see A278545, for a square spiral see A010731; and for a hexagonal spiral see A010722.

Links

Table of n, a(n) for n=1..91.

Example

The sequence written as an isosceles triangle begins:
.
.                     2;
.                   4,  4;
.                 4,  6,  4;
.               4,  6,  6,  4;
.             4,  6,  6,  6,  4;
.           4,  6,  6,  6,  6,  4;
.         4,  6,  6,  6,  6,  6,  4;
.       4,  6,  6,  6,  6,  6,  6,  4;
.     4,  6,  6,  6,  6,  6,  6,  6,  4;
.   4,  6,  6,  6,  6,  6,  6,  6,  6,  4;
...

Crossrefs

Row sums give A016933.

Left border gives A040002, the same as the right border.

Middle column gives the elements > 1 of A134201, also twice A122553.

Cf. A010722, A010731, A275015, A278317, A278480, A278545.

Sequence in context: A284560 A035633 A160803 * A084291 A258573 A073806

Adjacent sequences: A278478 A278479 A278480 * A278482 A278483 A278484

Keyword

nonn,tabl

Author

Omar E. Pol, Nov 23 2016

Status

approved