The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation. Please make a donation to keep the OEIS running. We are now in our 56th year. In the past year we added 10000 new sequences and reached almost 9000 citations (which often say "discovered thanks to the OEIS"). Other ways to donate

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A057247 a(n) is the smallest prime of the form 1 + prime(n)*2^m. 1
 5, 7, 11, 29, 23, 53, 137, 1217, 47, 59, 7937, 149, 83, 173 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The prime a(15) has 449 decimal digits (see A058887). LINKS Jean-François Alcover, Table of n, a(n) for n=16..50. FORMULA a(n) = Min{q|q is prime, p(n) is the n-th prime and q = 1+p(n)*2^b(n)}. EXAMPLE Sophie-Germain primes are here at n = 1, 2, 3, 5, 9, 10, .. etc. At n = 11, p(11) = 31 and in the sequence of q = 1+31*{2, 4, 8, 16, 32, 64, 128, 256} = {63, 125, 249, 497, 993, 1985, 3969, 7937}, the first prime is 7937, so b(11) = 8, a(11) = 7937. MATHEMATICA a[n_] := (For[pn = Prime[n]; p = 2, p < 3*10^8 (* large enough to compute 50 terms except a(15) *), p = NextPrime[p], m = Log[2, (p-1)/pn]; If[m > 0 && IntegerQ[m], Print["a(", n, ") = ", p]; Return[p]]]; Print["a(", n, ") not found ", p]; 0); Table[a[n], {n, 1, 50}] (* Jean-François Alcover, Nov 08 2016 *) CROSSREFS Cf. A058887, A005384, A005385. Sequence in context: A036491 A036490 A106330 * A157437 A213677 A330775 Adjacent sequences:  A057244 A057245 A057246 * A057248 A057249 A057250 KEYWORD nonn AUTHOR Labos Elemer, Jan 10 2001 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified December 2 03:22 EST 2020. Contains 338865 sequences. (Running on oeis4.)