OFFSET
1,3
LINKS
Antti Karttunen, Table of n, a(n) for n = 1..16384
Antti Karttunen, Data supplement: n, a(n) computed for n = 1..100000
FORMULA
a(n) >= A175663(n). - Antti Karttunen, Jan 03 2019
EXAMPLE
For n=5, there is a run of primes from 5-A002110(1) to 5+A002110(2): [3, 5, 7, 11] (while 5-A002110(2) = 5-6 = -1 and 5+A002110(3) = 5+30 = 35 are not primes), thus a(5) = 4. - Antti Karttunen, Jan 03 2019
MATHEMATICA
Array[If[PrimeQ@ #, Block[{s = {1}, t = {1}}, While[PrimeQ[# + Times @@ Prime@ s], AppendTo[s, s[[-1]] + 1]]; While[And[# > 0, PrimeQ[#]] &[# - Times @@ Prime@ t], AppendTo[t, t[[-1]] + 1]]; Last[s] + Last[t] - 1], 0] &, 105] (* Michael De Vlieger, Jan 03 2019 *)
PROG
(PARI) A175682(n) = if(!isprime(n), 0, my(pr=2, dn=1, dp=1, rl=1); for(k=1, oo, if(!isprime(n-pr), dn=0); if(!isprime(n+pr), dp=0); if(!(dn+dp), return(rl)); rl += (dn+dp); pr *= prime(1+k))); \\ (Note that isprime in PARI/GP 2.9.4 returns 0 on all negative arguments) - Antti Karttunen, Jan 03 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Vladislav-Stepan Malakovsky and Juri-Stepan Gerasimov, Aug 08 2010
EXTENSIONS
Terms a(5), a(7) and a(19) corrected by Antti Karttunen, Jan 03 2019
STATUS
approved