first(n) = {my(res = vector(n, i, oo), todo = n-1); res[1] = 1;   
for(i = 2, oo, 
	c = maxpower(i); 
	if(c <= n && res[c] == oo, 
		todo--; 
		res[c] = i; 	
		if(todo == 0, 
			return(res)
		)
	)
)
	
}

maxpower(n) = {
my(f = factor(n), res = oo);
for(i = 1, #f~,
	res = min(res, val(n, f[i, 1]) \ f[i, 2])
); res
}

addhelp(val, "exponent of prime p in n!")
val(n, p) = my(r=0); while(n, r+=n\=p);r