OFFSET
1,5
COMMENTS
Van Eck's sequence (A181391) is an integer sequence defined as follows; Let a(0) = 0. For, n >= 0, if there exists an m < n such that a(m) = a(n), take the largest such m and set a(n+1) = n - m, otherwise a(n+1) = 0. This sequence modifies the Van Eck sequence, adding an additional condition in that if a(n) is prime, set a(n+1) = n - m, where m is the largest possible value such that a(m) is also prime.
EXAMPLE
Start with a(1) = 0.
0 has not occurred before, so the rule says a(2) = 0.
Now 0 HAS occurred before, at a(1), 0 is not prime, so a(3) = 1, which is the number of places before a(2) where 0 occurs.
1 has not occurred before, so a(4) = 0.
0 occurred most recently at term a(2) and is not prime, so a(5) = 2.
2 has not occurred before, so a(6) = 0.
0 occurred most recently at term a(4) and is not prime, so a(7) = 2.
2 occurred most recently at term a(5) and 2 IS prime, so a(8) = 2 which is the number of places back where the previous prime occurred.
.. and so on!
PROG
(PARI) lista(nn) = {my(t, v=vector(nn)); for(n=2, nn-1, if(vecsearch(vecsort(v[1..n-1]), v[n]), t=1; if(isprime(v[n]), while(!isprime(v[n-t]), t++), while(v[n-t]!=v[n], t++)); v[n+1]=t)); v; } \\ Jinyuan Wang, Aug 20 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
Philip Mizzi, Aug 07 2020
EXTENSIONS
More terms from Jinyuan Wang, Aug 07 2020
STATUS
approved