The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 56th year, we are closing in on 350,000 sequences, and we’ve crossed 9,700 citations (which often say “discovered thanks to the OEIS”).

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A059456 Unsafe primes: primes not in A005385. 17
 2, 3, 13, 17, 19, 29, 31, 37, 41, 43, 53, 61, 67, 71, 73, 79, 89, 97, 101, 103, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 173, 181, 191, 193, 197, 199, 211, 223, 229, 233, 239, 241, 251, 257, 269, 271, 277, 281, 283, 293, 307, 311, 313, 317, 331, 337 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS A010051(a(n))*(1-A156659(a(n))) = 1; subsequence of A156657. - Reinhard Zumkeller, Feb 18 2009 Also, primes p such that p-1 is a non-semiprime. - Juri-Stepan Gerasimov, Apr 28 2010 Conjecture: From the sequence of prime numbers, let 2 and remove the first data iteration of 2*p+1; leave 3 and remove the prime data by the iteration 2*p+1 and we get the sequence. Example for p=2, remove(5,11,23,47); p=3, remove(7); p=13, p=17, p=19, p=23, remove(47); and so on. - Vincenzo Librandi, Aug 07 2010 LINKS Michael De Vlieger, Table of n, a(n) for n = 1..10000 EXAMPLE 31 is here because (31-1)/2=15 is not prime. 2 and 3 are here because 1/2 and 1 are not prime numbers. MATHEMATICA Complement[Prime@ Range@ PrimePi@ Max@ #, #] &@ Select[Prime@ Range@ 90, PrimeQ[(# - 1)/2] &] (* Michael De Vlieger, May 01 2016 *) Select[Prime[Range[100]], PrimeOmega[#-1]!=2&] (* Harvey P. Dale, May 13 2018 *) PROG (PARI) is(n)=isprime(n) && !isprime(n\2) \\ Charles R Greathouse IV, May 02 2016 CROSSREFS Cf. A005384, A005385, A053176, A059452-A059456, A007700, A005602, A023272, A023302, A023330. Initial terms for groups in A075712. Sequence in context: A299543 A059670 A038975 * A274589 A019394 A048704 Adjacent sequences:  A059453 A059454 A059455 * A059457 A059458 A059459 KEYWORD nonn AUTHOR Labos Elemer, Feb 02 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 7 13:08 EST 2021. Contains 349581 sequences. (Running on oeis4.)