A335496 a(n) is the least odd number k such that Omega(k) = n, Omega(k+2) = n+1, and Omega(k+4) = n+2, where Omega(k) is the number of prime factors of k (A001222).

23, 871, 5423, 229955, 13771373, 558588875, 21549990623, 1325878234371, 17040894859373, 429205867309373

Offset

1,1

Comments

a(n) mod 81 for n = 1..8: {23, 61, 77, 77, 77, 77, 77, 0}.

Links

Table of n, a(n) for n=1..10.

Formula

a(n) >= A335498(n). - Daniel Suteu, Jul 08 2023

Example

23 is a term: 23 is a prime, 25=5*5 is a semiprime, 27=3*3*3 is a triprime.
871 is a term: 871 = 13*67 (semiprime), 873 = 3*3*97 (triprime), 875 = 5*5*5*7 (quadprime).

Prog

(PARI)
generate(A, B, n, k) = A=max(A, 2^n); (f(m, p, n) = my(list=List()); if(n==1, forprime(q=max(p, ceil(A/m)), B\m, my(t=m*q); if(bigomega(t-4) == k && bigomega(t-2) == k+1, listput(list, t-4))), forprime(q=p, sqrtnint(B\m, n), list=concat(list, f(m*q, q, n-1)))); list); vecsort(Vec(f(1, 3, n)));
a(n) = my(x=2^n, y=2*x); while(1, my(v=generate(x, y, n+2, n)); if(#v >= 1, return(v[1])); x=y+1; y=2*x); \\ Daniel Suteu, Jul 08 2023

Crossrefs

Cf. A335498.

Sequence in context: A158505 A217116 A081731 * A183523 A233339 A008961

Adjacent sequences: A335493 A335494 A335495 * A335497 A335498 A335499

Keyword

nonn,more

Author

Zak Seidov and Giovanni Resta, Jun 11 2020

Extensions

a(9)-a(10) from Daniel Suteu, Jul 08 2023

Status

approved