A101985 Numbers that occur exactly once in A289493 (= number of primes between 2n and 3n).

11, 42, 93, 110, 113, 156, 186, 196, 197, 220, 252, 292, 298, 362, 403, 429, 493, 503, 609, 644, 659, 727, 735, 778, 790, 886, 888, 920, 932, 952, 953, 1008, 1023, 1024, 1079, 1093, 1094, 1100, 1109, 1136, 1165, 1208, 1212, 1213, 1226, 1238, 1250, 1311

Offset

1,1

Links

Table of n, a(n) for n=1..48.

Mathematica

f[n_] := PrimePi[3n] - PrimePi[2n]; t = Split[ Sort[ Table[ f[n], {n, 14000}] ]]; Flatten[ Select[t, Length[ # ] == 1 &]] (* Robert G. Wilson v, Feb 10 2005 *)

Prog

(PARI) bet2n3n(n)={ my(b=vecsort( vector(n, x, my(c=0); forprime(y=2*x+1, 3*x-1, c++); c))); for(x=1, n-2, if(b[x+1]>b[x] && b[x+1]<b[x+2], print1(b[x+1]", ")))} \\ Probably using A289493 and/or primepi(3n)-primepi(2n) would be faster. Edited and corrected by M. F. Hasler, Sep 29 2019
(PARI) \\ Size of vector dependent on how pessimistic one is on smoothness of primepi
primecount(a, b)=primepi(b)-primepi(a);
v=vector(14000);
for(k=1, oo, j=primecount(2*k, 3*k); if(j>#v, break, v[j]++));
for(k=1, 1311, if(v[k]==1, print1(k, ", "))) \\ Hugo Pfoertner, Sep 29 2019

Crossrefs

Cf. A289493, A101984.

Sequence in context: A003356 A063152 A338627 * A055437 A055436 A213772

Adjacent sequences: A101982 A101983 A101984 * A101986 A101987 A101988

Keyword

easy,nonn

Author

Cino Hilliard, Jan 29 2005

Extensions

More terms from Robert G. Wilson v, Feb 10 2005

Name edited by M. F. Hasler, Sep 29 2019

Status

approved