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 A225781 Numbers k such that both k and (k+1)/2 are primes and evil. 1
 5, 277, 673, 1093, 1237, 1381, 1621, 1873, 2473, 2593, 2797, 2857, 4177, 4357, 4441, 4561, 4933, 5077, 5233, 5413, 5437, 5581, 5701, 6037, 6133, 6997, 7477, 7537, 8053, 8353, 8713, 8893, 9133, 9901, 10861, 10957, 11113, 11161, 11497, 12073, 12457, 12757 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS It seems to be the case that all primes k where (k+1)/2 is also prime share the property that they are also both either evil or odious, the sole exception being 3, which is evil but has 2 as an odious companion. The last comment is true; for k and (k+1)/2 to be prime, k must be the number 3 or have the form 4*m + 1. The latter means its binary expansion ends in 01. Adding 1 to such a number and dividing by 2 leaves the bit count the same. Hence, both of these numbers have the same parity; they are both evil or both odious. - Jon Perry, May 25 2013 LINKS Brad Clardy, Table of n, a(n) for n = 1..1000 MATHEMATICA evilQ[n_] := EvenQ[DigitCount[n, 2, 1]]; Select[Prime[Range[1600]], PrimeQ[(#+1)/2] && And @@ evilQ /@ {#, (#+1)/2} &] (* Amiram Eldar, Aug 06 2023 *) PROG (Magma) //the function Bweight determines the binary weight of a number Bweight := function(m) Bweight:=0; adigs := Intseq(m, 2); for n:= 1 to Ilog2(m)+1 do Bweight:=Bweight+adigs[n]; end for; return Bweight; end function; for i:=1 to 1000000 do pair:=(i+1)div 2; if (IsPrime(i) and IsPrime(pair) and (Bweight(i) mod 2 eq 0) and (Bweight(pair) mod 2 eq 0)) then i; end if; end for; CROSSREFS Cf. A005383 (both k and (k+1)/2 are primes), A001969 (evil numbers). Sequence in context: A213958 A158115 A260197 * A368754 A057209 A216662 Adjacent sequences: A225778 A225779 A225780 * A225782 A225783 A225784 KEYWORD nonn,base AUTHOR Brad Clardy, May 15 2013 STATUS approved

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Last modified May 26 03:59 EDT 2024. Contains 372807 sequences. (Running on oeis4.)