A397165 Apply PrimePi to A397164.

33, 120, 171, 201, 203, 210, 213, 230, 238, 264, 286, 288, 294, 322, 354, 373, 395, 479, 486, 506, 542, 553, 563, 583, 609, 628, 646, 659, 688, 700, 719, 724, 730, 744, 749, 770, 814, 862, 883, 963, 975, 1009, 1020, 1056, 1103, 1115, 1119, 1182, 1247, 1260, 1273, 1324, 1358

Offset

1,1

Comments

These are the indices of the primes in A280864 that begin a block of length 4 that both begins and ends with a prime.

Links

Michael De Vlieger, Table of n, a(n) for n = 1..2856

Mathematica

Block[{c, j, k, m, q, r, s, t, w, nn, rad},
  nn = 24000; c[_] := False; q = 0; r = s = 1; t = {};
  rad[n_] := rad[n] = Times @@ FactorInteger[n][[All, 1]];
  Monitor[Reap[Do[w = GCD[r, s]; m = r/w; k = m;
    While[Or[c[k], ! CoprimeQ[w, k]], k += m];
      c[k] = True; s = r; r = rad[k];
    If[CoprimeQ[s, r],
      If[Length[t] == 4,
        If[AllTrue[{First[t], Last[t]}, PrimeQ], Sow[PrimePi@ First[t] ] ] ];
      q++; t = {k}, AppendTo[t, k] ], {i, nn}] ][[-1, 1]], i] ] (* Michael De Vlieger, Jun 17 2026 *)

Crossrefs

Cf. A000720, A280774, A280864, A283832, A396300, A396301, A397102, A397164.

Sequence in context: A140161 A177211 A337626 * A301633 A039440 A235902

Adjacent sequences: A397162 A397163 A397164 * A397166 A397167 A397168

Keyword

nonn,new

Author

N. J. A. Sloane, Jun 17 2026

Status

approved