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 A324858 Numbers m > 1 such that there exists a composite divisor c of m with s_c(m) = c. 3
 28, 40, 52, 66, 76, 88, 96, 100, 112, 120, 126, 136, 148, 153, 156, 160, 176, 186, 190, 196, 208, 225, 232, 246, 268, 276, 280, 288, 292, 297, 304, 306, 328, 336, 340, 344, 352, 366, 369, 370, 378, 388, 396, 400, 408, 435, 441, 448, 456, 460, 486, 496, 513, 516, 520, 532, 540, 544, 546, 550, 560, 568, 576, 580, 585, 592 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The function s_c(m) gives the sum of the base-c digits of m. The main entry for this sequence is A324456 = numbers m > 1 such that there exists a divisor d > 1 of m with s_d(m) = d. It appears that d is usually prime: compare the subsequence A324857 = numbers m > 1 such that there exists a prime divisor p of m with s_p(m) = p. However, d is usually composite for higher values of m. For any composite c, 0 < b < c, and 0 < i < j, b*c^i + (c-b)*c^j is in the sequence. - Robert Israel, Mar 19 2019 The sequence does not contain the 3-Carmichael numbers A087788, but intersects the Carmichael numbers A002997 that have at least four factors. This is a nontrivial fact. Examples for such Carmichael numbers below one million: 41041 = 7*11*13*41, 172081 = 7*13*31*61, 188461 = 7*13*19*109, 278545 = 5*17*29*113, 340561 = 13*17*23*67, 825265 = 5*7*17*19*73. For further properties of the terms see A324456 and Kellner 2019. - Bernd C. Kellner, Apr 02 2019 LINKS Robert Israel, Table of n, a(n) for n = 1..10000 Bernd C. Kellner, On primary Carmichael numbers, arXiv:1902.11283 [math.NT], 2019. EXAMPLE s_4(28) = 4 as 28 = 3 * 4 + 1 * 4^2, so 28 is a member. MAPLE S:= proc(c, m) convert(convert(m, base, c), `+`) end proc: filter:= proc(m) ormap(c -> (S(c, m)=c), remove(isprime, numtheory:-divisors(m) minus {1})) end proc: select(filter, [\$1..1000]); # Robert Israel, Mar 19 2019 MATHEMATICA s[n_, b_] := If[n < 1 || b < 2, 0, Plus @@ IntegerDigits[n, b]]; f[n_] := AnyTrue[Divisors[n], CompositeQ[#] && s[n, #] == # &]; Select[Range[600], f[#] &] (* simplified by Bernd C. Kellner, Apr 02 2019 *) PROG (PARI) isok(n) = {fordiv(n, d, if ((d>1) && !isprime(d) && (sumdigits(n, d) == d), return (1)); ); } \\ Michel Marcus, Mar 19 2019 CROSSREFS A324456 is the union of A324857 and A324858. Sequence in context: A195897 A109798 A216594 * A084807 A184032 A361855 Adjacent sequences: A324855 A324856 A324857 * A324859 A324860 A324861 KEYWORD nonn,base AUTHOR Jonathan Sondow, Mar 17 2019 STATUS approved

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Last modified December 10 04:38 EST 2023. Contains 367699 sequences. (Running on oeis4.)