A125838 - OEIS

%I #9 Mar 29 2017 19:43:13

%S 2,2,2,6,120,120,2894220,397073040,1236161850,764907546690,

%T 8955490023480,138393712627170,8047290924923250

%N a(n) is the smallest number m such that k*m - 1 for k=2,3,...,n is prime.

%H Carlos Rivera, <a href="http://www.primepuzzles.net/puzzles/puzz_379.htm">Problems & Puzzles</a>.

%e a(7)=120 because 2*120-1, 3*120-1, 4*120-1, 5*120-1, 6*120-1 & 7*120-1 are prime and 120 is the smallest such number.

%t a[n_] := Block[{k=2}, While[! AllTrue[k Range[2, n] - 1, PrimeQ], k += 2]; k]; a /@

%t Range[2, 8] (* _Giovanni Resta_, Mar 29 2017 *)

%Y Cf. A125839, A088651, A088250.

%K hard,more,nonn

%O 2,1

%A _Carlos Rivera_, Jan 01 2007

%E a(8) from Luke Pebody (luke.pebody(AT)gmail.com)

%E a(9)-a(10) from Vladimir Trushkov (vladimir(AT)trushkov.botik.ru)

%E a(11) from _Farideh Firoozbakht_

%E a(5) corrected by and a(12)-a(14) from _Giovanni Resta_, Mar 29 2017