OFFSET
0,4
COMMENTS
LINKS
Sébastien Palcoux, Table of n, a(n) for n = 0..10000
PROG
(PARI) a=[]; for(n=1, 190, a=concat(a, 1); while(hasAP(a, 4), a[#a]++)); a \\ See A248625 for hasAP().
(SageMath)
cpdef FourFree(int n):
cdef int i, r, k, s, L1, L2, L3
cdef list L, Lb
cdef set b
L=[1, 1, 1]
for k in range(3, n):
b=set()
for i in range(k):
if 3*((k-i)/3)==k-i:
r=(k-i)/3
L1, L2, L3=L[i], L[i+r], L[i+2*r]
s=3*(L2-L1)+L1
if s>0 and L3==2*(L2-L1)+L1:
b.add(s)
if 1 not in b:
L.append(1)
else:
Lb=list(b)
Lb.sort()
for t in Lb:
if t+1 not in b:
L.append(t+1)
break
return L
# Sébastien Palcoux, Aug 28 2019
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
M. F. Hasler, Oct 10 2014
STATUS
approved