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 A280879 Occurrences of decrease of the probability density P(n) of coprime numbers k,m, satisfying 1 <= k <= a(n) and 1 <= m <= a(n), and a(n) congruent to 1 (mod 2) and a(n) not congruent to 3 (mod 6). 2
 5005, 6545, 7315, 7735, 8645, 8855, 10465, 11165, 11935, 14245, 25025, 32725, 35035, 36575, 38675, 43225, 44275, 45815, 51205, 52325, 54145, 55055, 55825, 59675, 60515, 61985, 65065, 71225, 71995, 73255, 78155, 80465, 83545, 85085, 95095, 97405, 99715 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Probability densities satisfying P(a(n)) < P(a(n)-1) and a(n) congruent to 1 (mod 2) and a(n) not congruent to 3 (mod 6). It appears that most numbers satisfy a(n) congruent to 35 (mod 70), but a(74) congruent to 15 (mod 70) and a(93) congruent to 55 (mod 70). Subset of A280877. P(n) = ((2*Sum_{m=1..a(n)} phi(m))-1)/a(n)^2 (Cf. Euler phi function A000010). LINKS Chai Wah Wu, Table of n, a(n) for n = 1..10000 (terms 1..102 from A.H.M. Smeets) PROG (Python) from fraction import gcd t = 1 to = 1 i = 1 x = 1 while x > 0: ....x = x + 1 ....y = 0 ....while y < x: ........y = y + 1 ........if gcd(x, y) == 1: ............t = t + 2 ....e = t*(x-1)*(x-1) - to*x*x ....if (e < 0 and x%2 == 1 and x%6 != 3): ........print(i, x) ........i = i + 1 ....to = t (PARI) P(n) = (2 *sum(j=1, n, eulerphi(j)) - 1)/n^2; isok(n) = (n % 2) && ((n % 6) != 3) && (P(n) < P(n-1)); \\ Michel Marcus, Jan 29 2017 (Python) from sympy import totient A280879_list, n, t = [], 1, 1 while len(A280879_list) < 1000:     n += 1     h = totient(n)     t2 = t+h     if n % 2 and n % 6 != 3 and 2*(n*(h*n - 2*t2 + 1) + t2) <  1:         A280879_list.append(n)     t = t2 # Chai Wah Wu, Feb 11 2017 CROSSREFS Cf. A018805, A280877, A280878. Sequence in context: A035901 A108011 A018188 * A067226 A249081 A154059 Adjacent sequences:  A280876 A280877 A280878 * A280880 A280881 A280882 KEYWORD nonn AUTHOR A.H.M. Smeets, Jan 09 2017 STATUS approved

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Last modified April 3 17:03 EDT 2020. Contains 333197 sequences. (Running on oeis4.)