1,1

Sequence gives values of n such sum(i=1,n,GCD(n,i))=A018804(n) is prime. - Benoit Cloitre (benoit7848c(AT)orange.fr), Jan 25 2002

Let q = 2n-1. For these n (and q), the sum of two cyclotomic polynomials can be written as a product of cyclotomic polynomials and as a cyclotomic polynomial in x^2: Phi(q,x) + Phi(2q,x) = 2 Phi(n,x) Phi(2n,x) = 2 Phi(n,x^2). - T. D. Noe (noe(AT)sspectra.com), Nov 04 2003

Numbers n such that the n-th Hexagonal number A000384(n) = n*(2*n-1) is semiprime. {A005382} = {n such that A000384(n) is an element of A001358}. - Jonathan Vos Post (jvospost3(AT)gmail.com), Feb 14 2006

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards Applied Math. Series 55, 1964 (and various reprintings), p. 870.

T. D. Noe, Table of n, a(n) for n=1..10000

M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards, Applied Math. Series 55, Tenth Printing, 1972 [alternative scanned copy].

J. Perry, Germain Primes

a(n) = A129521(n)/A005383(n). - Reinhard Zumkeller (reinhard.zumkeller(AT)gmail.com), Apr 19 2007

f := proc(Q) local t1, i, j; t1 := []; for i from 1 to 500 do j := ithprime(i); if isprime(2*j-Q) then t1 := [op(t1), j]; fi; od: t1; end; f(1);

Select[Prime[Range[300]], PrimeQ[2#-1]&]

Cf. A005383, A057326, A057327, A057328, A057329, A057330, A005603, A063908, A023204.

Cf. A000384, A001358.

Adjacent sequences: A005379 A005380 A005381 this_sequence A005383 A005384 A005385

Sequence in context: A074855 A038935 A073640 this_sequence A113165 A128025 A092064

nonn,easy

N. J. A. Sloane (njas(AT)research.att.com).