a(n) = {
	my(oz = 0, nz = 3); 
	while(nz <= n,
		oz = nz;
		nz = nextzero(nz);
	);
	binomial(n+1, 2) - binomial(oz + 1, 2)
}

nextzero(n) = {
	if(n == 0, return(3));
	my(c = binomial(n+1, 2), d = divisors(2*c));
	
	for(i = 2, #d, 
		if(d[i] > n && 
			if(binomial((d[i]), 2) - c > 0 && (binomial((d[i]), 2) - c)%(d[i]) == 0, 
				return(d[i]);
			);
		);
	);
}

first(n) = {
	n--; 
	my(res = [1..n]);
	res[1] = 1; 
	for(i = 2, n, 
		if(res[i-1] > 0,
			if(res[i-1]%i == 0,
				res[i] = 0
			,
				res[i] = res[i-1] + i;
			);
		,
			res[i] = res[i-1] + i
		);
	); 
	concat(0, res)
}

firstzeros(n) = {
	my(res = vector(n));
	for(i = 2, n, 
		res[i] = nextzero(res[i-1]);
	);
	res
}