1,2

{{{1,2,3},{4,5,6},{7,8,9}}, {{1,2,3},{4,6,8},{5,7,9}}, {{1,3,5},{2,4,6},{7,8,9}}, {{1,4,7},{2,5,8},{3,6,9}}, {{1,5,9},{2,3,4},{6,7,8}}}

are the 5 ways to split 1, 2, 3, ..., 9 into 3 arithemetical progressions each with 3 elements. Thus a(3)=5.

Cf. A104430-A104443.

Sequence in context: A107112 A051295 A009383 this_sequence A109319 A059219 A137533

Adjacent sequences: A104426 A104427 A104428 this_sequence A104430 A104431 A104432

nonn

Jonas Wallgren (jonwa(AT)ida.liu.se), Mar 17 2005