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A197504 Odd numbers m >= 3 for which phi(2*m)/2 = phi(m)/2 is odd, where phi = A000010 (Euler's totient), and the number 1 is included. 4
1, 3, 7, 9, 11, 19, 23, 27, 31, 43, 47, 49, 59, 67, 71, 79, 81, 83, 103, 107, 121, 127, 131, 139, 151, 163, 167, 179, 191, 199, 211, 223, 227, 239, 243, 251, 263, 271, 283, 307, 311, 331, 343, 347, 359, 361, 367, 379, 383, 419, 431, 439, 443, 463, 467, 479, 487, 491, 499, 503, 523, 529 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

The old name was A196445(n)/2.

From Wolfdieter Lang, Nov 12 2019: (Start)

These are the odd numbers m for which the degree of the algebraic number sin(Pi/(2*m)) (the degree of its minimal polynomial), given by A055035(2*m), is odd. Because A055035(1) = 1, there is just this other instance with odd A055035.

This sequence {a(n)} consists for n >= 2 of all powers >= 1 of each prime p == 3 (mod 4) from A002145, sorted into increasing order. This follows from the factorization of odd m >= 3, and that phi(m)/2 has to be odd.

For a(n), with n >= 2, there is exactly one pair of solutions x = +1 and -1 (the trivial solution) of the congruence x^2 == +1 (mod a(n)), and there is no solution of the congruence x^2 == -1 (mod a(n)). The proof starts with showing this for p == 3 (mod 4). It can be shown that the square 1 appears only for x = 1 if x runs through 1, ..., (p-1)/2. The other x range (p+1)/2, ..., p-1, which has the same squares (mod p), can, by reading backwards, be interpreted as the -x partners of x from the first range  The Legendre symbol (-1, p) = -1 shows the second claim. Then one applies the lifting theorem for powers of primes (see Apostol, Theorem 5.30, p. 121), where only its part (a) is needed, and the step by step lifting to each prime power is unique.

For a(1) = 1 there is just one solution x = 0 of the congruence x^2 == +1 (mod 1), and also of x^2 == -1 (mod 1).

The complementary sequence with odd m >= 3 and even phi(m)/2 is given in A327922.

(End)

REFERENCES

T. M. Apostol, Introduction to Analytic Number Theory, Springer-Verlag, 1976, pp. 121-122.

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

FORMULA

a(1) = 1, and for n >= 2 the set of all positive powers of each prime p == 3 (mod 4) (A002145), sorted increasingly.

EXAMPLE

Factorization for n >= 2:  3, 7, 3^2, 11, 19, 23, 3^3, 31, 43, 47, 7^2, 59, 67, 71, 79, 3^4, 83, 103, 107. ... - Wolfdieter Lang, Nov 12 2019

MATHEMATICA

a[n_] := If[n == 2, 1, EulerPhi[n]/{1, 1, 2, 1}[[Mod[n, 4] + 1]]]; aa = {}; Do[If[OddQ[a[n]], AppendTo[aa, n/2]], {n, 2, 1000}]; aa

CROSSREFS

Cf. A000010, A055035, A196445, A327922.

Sequence in context: A129747 A004614 A112398 * A167800 A270834 A192628

Adjacent sequences:  A197501 A197502 A197503 * A197505 A197506 A197507

KEYWORD

nonn,easy

AUTHOR

Artur Jasinski, Oct 15 2011

EXTENSIONS

Name changed by Wolfdieter Lang, Nov 12 2019

STATUS

approved

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Last modified July 2 14:41 EDT 2020. Contains 335401 sequences. (Running on oeis4.)