OFFSET
1,3
COMMENTS
Shortest SNUSP representation of a number using only + and @ is in A172005.
SNUSP is a programming language where each command is an individual letter. The three of concern here are + and @ and #. + increments the current data value, @ is a "subroutine call" and # is a "return". When an @ is encountered, a record of the location is put on a stack and execution continues. When a # is encountered, if there is a return point on the stack, the execution continues at that a single character beyond that return point. If there is no return point on the stack, execution terminates.
Thus "@@++#" would put the first two "@" return points on the stack, increment data twice, return from the second "@" to the last "+", increment the data once more, return from the first "@" to the first "+", increment the data two more times and finally terminate when it hits the "#" with no return points on the stack. The data is always initialized to zero so this effectively puts 5 into the data. In order to place a particular value into the data, there are one or more minimal strings of these characters. The i-th term of the sequence gives the number of minimal SNUSP programs using only these characters. After 2, all sequences end in either +++ or @++, which are equivalent, so all values above a(2) are even.
EXAMPLE
19 can be represented minimally in 6 ways using @, + and #: @+@+++++# +@@@++++# @++@@+++# @+@++@++# +@@@+@++# @++@@@++#. Thus a(19) = 6.
MAPLE
See A172005.
CROSSREFS
KEYWORD
nonn
AUTHOR
Darrell Plank (jar_czar(AT)msn.com), Jan 22 2010
STATUS
approved