A396673 a(n)=s is the first occurrence of 3 consecutive odd squarefree semiprimes r, s, t (A046388) such that gcd(r,s,t) = prime(n).

21, 58895, 4178503, 141828181, 2679208597, 166157155763, 145260505801, 8337472698931, 98744691199909, 787806046737133

Offset

2,1

Links

Table of n, a(n) for n=2..11.

Example

  n prime(n)       r      a(n)=s           t
  2    3          15          21          33
  3    5       58885       58895       58915
  4    7     4178489     4178503     4178531
  5   11   141828137   141828181   141828203
  6   13  2679208571  2679208597  2679208649

Prog

(PARI) a396673(n) = if(n<2, return(-1)); my(p=prime(n), p1=15, p2=21); forstep(k=33, oo, 2, my(f=factor(k)); if(omega(f)==2 && bigomega(f)==2, p3=k; if(gcd([p1, p2, p3])==p, return(p2)); p1=p2; p2=p3));

Crossrefs

Cf. A046388.

Sequence in context: A351804 A283841 A100414 * A189310 A068735 A172800

Adjacent sequences: A396670 A396671 A396672 * A396674 A396675 A396676

Keyword

nonn,hard,more,changed

Author

Hugo Pfoertner, Jun 02 2026

Extensions

a(10)-a(11) from Karl-Heinz Hofmann, Jun 10 2026

Status

approved