1,1

This sequence is different from A070826 and A118750.

Contribution from Enoch Haga (Enokh(AT)comcast.net), Jul 02 2009: (Start)

A clearer definition of the sequence: a(n) is the smallest odd composite

number m such that m - 2 is prime and set of the distinct prime factors of m

equals the set of the first n odd primes. Farideh Firoozbakthkt, Jun 30, 2009

(End)

Charles R Greathouse IV, Table of n, a(n) for n=1..200

Compute N = product of the first n odd primes. If N-2 is prime, add N to the sequence. Otherwise test 3N, 5N, 7N, 9N, ... until kN - 2 is prime, subject to A006530(k) <= n+1.

The first odd prime is 3. 3-1 is not prime, but 3*3-2 = 7 is prime so a(1) = 9. The product of the first two odd primes is 15, and 15-2 is prime, so a(2) = 15.

a[n_]:=(c=Product[Prime[k], {k, 2, n+1}]; For[m=1, !(!PrimeQ[c (2m-1)]&&PrimeQ[c(2m-1)-2]&&Length[FactorInteger[c(2m-1)]]==n), m++ ]; c(2m-1)); Table[a[n], {n, 20}] [From Enoch Haga (Enokh(AT)comcast.net), Jul 02 2009]

Cf. A136349-A136352 A136354-A136358 A070826 A118750.

Sequence in context: A100663 A152219 A029712 this_sequence A136354 A098146 A124274

Adjacent sequences: A136350 A136351 A136352 this_sequence A136354 A136355 A136356

nonn

Enoch Haga (Enokh(AT)comcast.net), Dec 25 2007

More terms, better title, and Mathematica program from Farideh Firoozbakht received Jun 30, 2009. - Enoch Haga (Enokh(AT)comcast.net), Jul 02 2009

Further editing by Charles R Greathouse IV (charles.greathouse(AT)case.edu), Oct 05 2009