OFFSET
1,1
EXAMPLE
11 is the first number that does not equal a count of primes between 2n and 4n for some n.
MATHEMATICA
f[n_] := PrimePi[4n] - PrimePi[2n]; t = Union[ Table[ f[n], {n, 12000}]]; Complement[ Range[ t[[ -1]]], t] (* Robert G. Wilson v, Feb 10 2005 *)
PROG
(PARI) bet2n4n(n)={ my( b=vecsort(vector(n, x, my(c=0); forprime(y=2*x+1, 4*x-1, c++); c))); for(x=1, n-2, while(b[x+1]-b[x]>1, print1(b[x]++, ", ")))} \\ It's probably faster to use A101909 instead of forprime(...). Edited and corrected by M. F. Hasler, Sep 29 2019
(PARI) primecount(a, b)=primepi(b)-primepi(a);
v=vector(20000);
for(k=1, oo, j=primecount(2*k, 4*k); if(j>#v, break, v[j]++));
for(k=1, 2170, if(v[k]==0, print1(k, ", "))) \\ Hugo Pfoertner, Sep 29 2019
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Cino Hilliard, Jan 28 2005
EXTENSIONS
More terms from Robert G. Wilson v, Feb 10 2005
Name edited by M. F. Hasler, Sep 29 2019
STATUS
approved