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 A291127 Consider the zeros of the polynomial P(m,x) whose coefficients are the divisors of a number m. The sequence lists the numbers m such that P(m,x) contains at least two zeros that are purely imaginary numbers. 2
 6, 8, 10, 14, 15, 21, 22, 26, 27, 33, 34, 35, 38, 39, 42, 46, 51, 54, 55, 57, 58, 62, 65, 66, 69, 74, 77, 78, 82, 85, 86, 87, 88, 91, 93, 94, 95, 102, 104, 106, 110, 111, 114, 115, 118, 119, 122, 123, 125, 128, 129, 130, 133, 134, 136, 138, 141, 142, 143, 145 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS P(m,x) = Sum_{i=1..k} d(i)*x^(i-1) where d(1), d(2), ..., d(k) are the k divisors of m. The number of zeros of the polynomial P(n,x) is given by A032741(n). We observe that all the zeros of the polynomial are located in the unit circle. Conjecture: A032741(a(n))= p where p == 3 (mod 4), p prime. Examples: A032741(m) = 3 for m = 6, 8, 10, 14, 15, 21, 22, 26, 27, 33, ... A032741(m) = 7 for m = 42, 54, 66, 78, 88, ... A032741(m) = 11 for m = 156, 204, 228, 276, 294, 342, 348, 372, ... A032741(m) = 19 for m = 2544, 2832, 2928, 3216, 3408, 3504, 3792, ... A032741(m) = 23 for m = 24492, 25428, 26052, 26364, 26988, 27924, ... Except for the two purely imaginary zeros of P(m,x), it seems that the complex zeros are of the form u +- u*i where u is a real number. From Wolfdieter Lang, Nov 07 2017: (Start) P(m,x) is the row m polynomial of A027750 with increasing powers of x. The numbers m = 1 and m = prime obviously do not appear in this sequence. The composite numbers m belonging to irreducible polynomials P(m,x) over the integers given in A292226 also do not appear in this sequence. Moreover, the composite numbers m with factorizable P(m,x) without a factor of the type a*x^2 + b, with positive integers a and b, also do not appear in this sequence; these are the numbers 18, 20, 28, 32, 44, ... (End) Are there numbers m with more than one pair of purely imaginary solutions? - Wolfdieter Lang, Nov 14 2017 From Robert Israel, Nov 14 2017: (Start) The even and odd parts of P(m,x) are of the form A(x^2) and x*B(x^2) for polynomials A and B with integer coefficients, and pairs of imaginary roots of P(m,x) correspond to negative roots of the gcd of A and B. Includes the following: p^k where p is prime and k==3 (mod 4). p*q^k where k is odd and p, q are prime with either p < q or p > q^k. p*q*r^k where p, q, r are distinct primes and r > p*q. (End) LINKS Robert Israel, Table of n, a(n) for n = 1..10000 EXAMPLE 42 is in the sequence because P(42,x) = 1 + 2x + 3x^2 + 6x^3 + 7x^4 + 14x^5 + 21x^6 + 42x^7 = (1 + 2*x)*(1 + 3*x^2)*(1 + 7*x^4), and the seven zeros are -1/2, +(1/3)*sqrt(3)*i, -(1/3)*sqrt(3)*i, r*(1+i), r*(1-i), r*(-1+i), r*(-1-i) with r = 7^(3/4)*sqrt(2)/14. The relevant factor for the two purely imaginary zeros is (1 + 3*x^2). - Wolfdieter Lang, Nov 13 2017 MAPLE F:= proc(n) local x, d, i, A, B, R; d:= sort(convert(numtheory:-divisors(n), list)); A:= add(d[2*i]*x^(i-1), i=1..nops(d)/2); B:= add(d[2*i+1]*x^i, i=0..(nops(d)-1)/2); R:= gcd(A, B); sturm(sturmseq(R, x), x, -infinity, 0) > 0; end proc: select(F, [\$1..1000]); # Robert Israel, Nov 14 2017 MATHEMATICA Position[#, k_ /; k >= 2][[All, 1]] &@ Table[Count[Re /@ Values@ Apply[Join, Solve[Normal@ SeriesData[x, 0, #, 0, Length@ #, 1] == 0, x]], 0] &@ Divisors@ n, {n, 150}] (* Michael De Vlieger, Aug 21 2017 *) PROG (PARI) isok(n) = {my(d = divisors(n), p = sum(k=1, #d, x^(k-1)*d[k])); #select(x->(real(x)==0), polroots(p)) >= 2; } \\ Michel Marcus, Sep 09 2017 CROSSREFS Cf. A027750, A032741, A292226. Sequence in context: A120497 A036436 A036455 * A211337 A007422 A030513 Adjacent sequences: A291124 A291125 A291126 * A291128 A291129 A291130 KEYWORD nonn AUTHOR Michel Lagneau, Aug 18 2017 STATUS approved

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