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 A216371 Odd primes with one coach: primes p such that A135303((p-1)/2) = 1. 8
 3, 5, 7, 11, 13, 19, 23, 29, 37, 47, 53, 59, 61, 67, 71, 79, 83, 101, 103, 107, 131, 139, 149, 163, 167, 173, 179, 181, 191, 197, 199, 211, 227, 239, 263, 269, 271, 293, 311, 317, 347, 349, 359, 367, 373, 379, 383, 389, 419, 421, 443, 461, 463, 467, 479, 487 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Given that prime p has only one coach, the corresponding value of k in A003558 must be (p-1)/2, and vice versa.  Using the Coach theorem of Jean Pedersen et al., phi(b) = 2 * c * k, with b odd.  Let b = p, prime.  Then phi(p) = (p-1), and k must be (p-1)/2 iff c = 1. Or, phi(p) = (p-1) = 2 * 1 * (p-1)/2. Conjecture relating to odd integers: iff an integer is in the set A216371 and is either of the form 4q - 1 or 4q + 1, (q>0); then the top row of its coach (Cf. A003558) is composed of a permutation of the first q odd integers. Examples: 11 is of the form 4q - 1, q = 3; with the top row of its coach [1, 5, 3]. 13 is of the form 4q + 1, q = 3; so has a coach of [1, 3, 5]. 37 is of the form 4q + 1, q = 9; so has a coach with the top row composed of a permutation of the first 9 odd integers: [1, 9, 7, 15, 11, 13, 3, 17, 5]. - Gary W. Adamson, Sep 08 2012 These are also the odd primes a(n) for which there is only one periodic Schick sequence (see the reference, and also the Brändli and Beyne link, eq. (2) for the recurrence but using various inputs. See also a comment in A332439). This sequence has primitive period length (named pes in Schick's book) A003558((a(n)-1)/2) = A005034(a(n)) = A000010(a(n))/2 = (a(n) - 1)/2, for n >=1. - Wolfdieter Lang, Apr 09 2020 REFERENCES P. Hilton and J. Pedersen, A Mathematical Tapestry, Demonstrating the Beautiful Unity of Mathematics, 2010, Cambridge University Press, pages 260-264. Carl Schick, Trigonometrie und unterhaltsame Zahlentheorie, Bokos Druck, Zürich, 2003 (ISBN 3-9522917-0-6). Tables 3.1 to 3.10, for odd p = 3..113 (with gaps), pp. 158-166. LINKS T. D. Noe and Charles R Greathouse IV, Table of n, a(n) for n = 1..10000 (first 1000 terms from T. D. Noe) Gerold Brändli and Tim Beyne, Modified Congruence Modulo n with Half the Amount of Residues, arXiv:1504.02757 [math.NT], 2016. FORMULA Odd primes p such that 2^m is not 1 or -1 mod p for 0 < m < (p-1)/2. - Charles R Greathouse IV, Sep 15 2012 a(n) = 2*A054639(n) + 1. - L. Edson Jeffery, Dec 18 2012 EXAMPLE Prime 23 has a k value of 11 = (23 - 1)/2 (Cf. A003558(11).  It follows that 23 has only one coach (A135303(11) = 1). 23 is thus in the set. On the other hand 31 is not in the set since A135303(15) shows 3 coaches, with A003558(15) = 5. 13 is in the set since A135303(6) = 1; but 17 isn't since A135303(8) = 2. MAPLE isA216371 := proc(n)     if isprime(n) then         if A135303((n-1)/2) = 1 then             true;         else             false;         end if;     else         false;     end if; end proc: A216371 := proc(n)     local p;     if n = 1 then         3;     else         p := nextprime(procname(n-1)) ;         while true do             if isA216371(p) then                 return p;             end if;             p := nextprime(p) ;         end do:     end if; end proc: seq(A216371(n), n=1..40) ; # R. J. Mathar, Dec 01 2014 MATHEMATICA Suborder[a_, n_] := If[n > 1 && GCD[a, n] == 1, Min[MultiplicativeOrder[a, n, {-1, 1}]], 0]; nn = 150; Select[Prime[Range[2, nn]], EulerPhi[#]/(2*Suborder[2, #]) == 1 &] (* T. D. Noe, Sep 18 2012 *) f[p_] := Sum[Cos[2^n Pi/((2 p + 1))], {n, p}]; 1 + 2 * Select[Range[500], Reduce[f[#] == -1/2, Rationals] &]; (* Gerry Martens, May 01 2016 *) PROG (PARI) is(p)=for(m=1, p\2-1, if(abs(centerlift(Mod(2, p)^m))==1, return(0))); p>2 && isprime(p) \\ Charles R Greathouse IV, Sep 18 2012 CROSSREFS Cf. A000010, A000040, A003558, A005034, A054639, A135303. Sequence in context: A095070 A079733 A179538 * A095747 A132779 A192869 Adjacent sequences:  A216368 A216369 A216370 * A216372 A216373 A216374 KEYWORD nonn AUTHOR Gary W. Adamson, Sep 05 2012 STATUS approved

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Last modified October 24 04:47 EDT 2020. Contains 337975 sequences. (Running on oeis4.)