3 ms·
https://oeis.org/A001105 https://oeis.org/A001105 starts with 0 so I guess you don't mean that obvious quadratic. Could be https://oeis.org/A209303 https://oei
by eesmith 4y ago
https://oeis.org/A001105 https://oeis.org/A001105 starts with 0 so I guess you don't mean that obvious quadratic.
Could be https://oeis.org/A209303 https://oeis.org/A209303 :
def f(n):
return n*n + sum(k*k for k in map(int, str(n)))
>>> [f(i) for i in range(1, 5)]
[2, 8, 18, 32]
but I don't know why that entry doesn't start with 0.
Could be https://oeis.org/A190787 https://oeis.org/A190787 :
>>> import heapq, itertools
>>> seq = heapq.merge((2**i for i in itertools.count(1, 2)),
(9*2**i for i in itertools.count(1, 2)))
>>> list(itertools.islice(seq, 0, 4))
[2, 8, 18, 32]
Or https://oeis.org/A067051 https://oeis.org/A067051:
import itertools
def sigma(n):
return sum(i for i in range(1, n//2+1) if n % i == 0)
def seq():
for n in itertools.count(1):
if sigma(2*n) % 2 == 0: continue
if sigma(3*n) % 3 == 0: yield n
continue
>>> list(itertools.islice(seq(), 0, 4))
[2, 8, 18, 32]
The others are either degenerate for 4 points of the A001105 case, or are not so easy to implement in raw Python.
- jacquesm 4y agoThink chemistry...
- eesmith 4y agoI believe you are referring to one of the many listed ways to define https://oeis.org/A001105 https://oeis.org/A001105 , which is even more succinctly expressed as a(n) = 2*n^2. This is the answer I thought you wanted, and which I was avoiding saying. My underlying point is your sequence was under-specified, and with no clear reason to pick your preferred answer over other answers. Even knowing it's chemistry, three finite sequences which match your specification are: 2, 8, 18, 32, 18, 8, 2 2, 8, 18, 32, 21, 9, 2 2, 8, 18, 32, 32, 14, 2