[seqfan] Re: A122036

Richard Mathar mathar at strw.leidenuniv.nl
Mon Mar 28 15:37:18 CEST 2011

This is the Maple function that checks entries in A122036;
hastily commented, hazardous if swallowed with an empty stomach etc:

# This function checks whether the argument n is a member of A005231
# it is if the sigma function of the argument is larger than 2n.
isA005231 := proc(n)
	is(numtheory[sigma](n) > 2*n );
end proc:
# recursive function which tests whether n can be written as a sum
# over a subset of elements of the set s
isA136446a := proc(s, n)
	if n in s then
		# if n is a member of s, the answer is yes (just take n)
		return true;
	elif add(i, i=s) < n then
		# if the sum over all elements of s is smaller than n,
		# the answer is no
		return false;
	elif nops(s) = 1 then
		# if there is exactly one element in s, the answer is yes
		# if that element equals n, otherwise the anise is no
		is(op(1, s)=n) ;
		# Put the set s in a sorted state
		sl := sort(convert(s, list), `>`) ;
		for i from 1 to nops(sl) do
			# assume that the maximum element of the subset that adds to n
			# is m and take m in decreasing order from the sorted list.
			m := op(i, sl) ;
			if n -m = 0 then return
				# if taking m exactly matches n, the answer is yes
				# (this is probably redundant as a check, see above)
			end if ;
			# If that largest m in the subset is not larger than n,
			# try to find other elements to complete n
			if n-m > 0 then
				# the recursive call: define the subset of elements
				# which are each smaller than n
				sr := [op(i+1..nops(sl), sl)] ;
				# .. and test whether (assuming m is taken) the remainder n-m
				# can be matched by another subset
				if procname(convert(sr, set), n-m) then
					return true;
				end if;
			end if;
		end do;
		return false;
	end if;
end proc:

# test whether the argument n is a member o fA136446
# return true if yes
isA136446 := proc(n)
	# this is done by taking the set of divisors without 1 and n,
	# testing whether n can be written as a sum of a subset of these
	isA136446a( numtheory[divisors](n) minus {1, n}, n) ;
end proc: 

# Main loop over all odd n
for n from 1 by 2 do
	#if n is in A005231 then check whether it is in A136446
	if isA005231(n) then
		if not isA136446(n) then
			printf("%d,\n",n) ;
		end if;
	end if;
	# print progress bars
	if n mod 150000 = 1 then
	end if;
end do:

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