OFFSET
2,1
COMMENTS
From Jon E. Schoenfield, Nov 15 2018: (Start)
If a(11) is odd, it is 16554218751.
If a(12) is odd, it is 105265530369.
If a(13) is odd, it is 1957645712385. (End)
a(11), a(12), and a(13) are indeed odd. - Giovanni Resta, Jan 04 2019
10^13 < a(14) <= 240455334218751, a(15) <= 2992278212890624. - Giovanni Resta, Jan 06 2019
EXAMPLE
For n = 3, 50 = 2*5*5, and the numbers before and after 50 are 49 = 7*7 and 51 = 3*17.
MATHEMATICA
a[n_] := Module[{o={0, 0, 0}, k=1}, While[o!={n-1, n, n-1}, o=Rest[AppendTo[o, PrimeOmega[k]]]; k++]; k-2]; Array[a, 7, 2] (* Amiram Eldar, Nov 14 2018 *)
PROG
(PARI) {for(n=2, 10, for(k=2^n, 10^12, if(n==bigomega(k) &&
n-1==bigomega(k-1) && n-1==bigomega(k+1), print1(k", "); break())))}
CROSSREFS
Cf. A078840.
Sequences listing r-almost primes, that is, the n such that A001222(n) = r: A000040 (r = 1), A001358 (r = 2), A014612 (r = 3), A014613 (r = 4), A014614 (r = 5), A046306 (r = 6), A046308 (r = 7), A046310 (r = 8), A046312 (r = 9), A046314 (r = 10), A069272 (r = 11), A069273 (r = 12), A069274 (r = 13), A069275(r = 14), A069276 (r = 15), A069277 (r = 16), A069278 (r = 17), A069279 (r = 18), A069280 (r = 19), A069281 (r = 20).
KEYWORD
nonn,more
AUTHOR
Zak Seidov, Nov 13 2018
EXTENSIONS
a(10) from Jon E. Schoenfield, Nov 14 2018
a(11)-a(13) from Giovanni Resta, Jan 04 2019
STATUS
approved