OFFSET
1,2
COMMENTS
Or say numbers which are "very difficult" to be typed on a keypad without the zero. (See description of A215009.)
LINKS
FUNG Cheok Yin, Table of n, a(n) for n = 1..453
FUNG Cheok Yin, C++ program
EXAMPLE
The keypad is:
+-----+
|1|2|3|
+-+-+-+
|4|5|6|
+-+-+-+
|7|8|9|
+-+-+-+
It is visibly obvious that 168 can be formed on the keypad, and each pairwise digits of 168 are not adjacent.
MATHEMATICA
no = IntegerDigits @ {12, 14, 23, 25, 34, 36, 45, 47, 56, 58, 69, 78, 89}; Sort[ FromDigits /@ Flatten[ Permutations /@ Select[ Subsets[ Range@ 9, {1, 9}], Intersection[ Subsets[#, {2}], no] == {} &], 1]] (* Giovanni Resta, Apr 06 2017 *)
CROSSREFS
KEYWORD
nonn,fini,full,base
AUTHOR
FUNG Cheok Yin, Mar 02 2017
STATUS
approved