OFFSET
1,2
COMMENTS
A subsequence of A010784. - FUNG Cheok Yin, Jul 05 2018
LINKS
FUNG Cheok Yin, Table of n, a(n) for n = 1..653
FUNG Cheok Yin, C++ program
EXAMPLE
The keypad is:
+---+---+---+
| 1 | 2 | 3 |
+---+---+---+
| 4 | 5 | 6 |
+---+---+---+
| 7 | 8 | 9 |
+---+---+---+
It is visibly obvious that 2589 can be formed on the keypad.
MATHEMATICA
g = Graph[{1 <-> 2, 1 <-> 4,
2 <-> 1, 2 <-> 3, 2 <-> 5,
3 <-> 2, 3 <-> 6,
4 <-> 1, 4 <-> 5, 4 <-> 7,
5 <-> 2, 5 <-> 4, 5 <-> 6, 5 <-> 8,
6 <-> 3, 6 <-> 5, 6 <-> 9,
7 <-> 4, 7 <-> 8,
8 <-> 5, 8 <-> 7, 8 <-> 9,
9 <-> 6, 9 <-> 8}];
f[{a_, b_}] := FindPath[g, a, b, Infinity, All]
ff = f /@ Flatten[Outer[List, r = Range[9], r], 1];
A280593 = Sort[Join[r, FromDigits /@ Flatten[ff, 1]]] (* Jean-François Alcover, Jan 06 2017 *)
CROSSREFS
KEYWORD
nonn,fini,full,base
AUTHOR
FUNG Cheok Yin, Jan 06 2017
STATUS
approved