OFFSET
1,3
COMMENTS
On a standard telephone keypad, i.e. 123 | 456 | 789 | 0, a number is "easy" to key in if each adjacent pair of digits in the number are the same, or are adjacent - either horizontally or vertically or diagonally. For example, adjacent to the 2 key are 1 and 3 (horizontal), 5 (vertical) and 4 and 6 (diagonal) so any of these digits could come after a 2 (but not 7, 8, 9 or 0).
LINKS
N. J. A. Sloane, Table of n, a(n) for n = 1..10001
EXAMPLE
126 is a term since the 1 and 2 keys are adjacent and the 6 is diagonally adjacent to the 2.
CROSSREFS
KEYWORD
nonn,dumb,base
AUTHOR
Paul Richards, Dec 12 2006
STATUS
approved