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A015865
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Numbers k such that sigma(k) = sigma(k+5).
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16
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6, 46, 1030, 2673, 4738, 4785, 10437, 14025, 20038, 20326, 23914, 28702, 31101, 39273, 39669, 41349, 41554, 44709, 46366, 55918, 68638, 74205, 93682, 94365, 96790, 103678, 115245, 115642, 124785, 169990, 182830, 185073, 207118, 214090
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OFFSET
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1,1
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COMMENTS
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Using the method proposed by Guy and Shanks to construct solutions of sigma(k) = sigma(k + 1), it is possible to search for large terms of this sequence: If q = 3^(m+1) + 8 and p = (3^m*q - 5)/2 are primes, then 2*p is a term. This occurs for m = 0, 1 and 4, giving the terms 6, 46 and 20326. If q = 3^(m+1) - 22 and p = (3^m*q + 5)/2 are primes, then 3^m*q is a term. This occurs for m = 45 giving the term 26183890704263137277609197558886063678754201. In both cases there are no other solutions for m <= 10^4. - Amiram Eldar, May 29 2020
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LINKS
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Donovan Johnson, Table of n, a(n) for n = 1..1000
Richard Guy and Daniel Shanks, A Constructed Solution of sigma(n) = sigma(n+1), The Fibonacci Quarterly, Vol. 12, No. 3 (1974), p. 299.
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MAPLE
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A015865:=proc(q) local n;
for n from 1 to q do if sigma(n+5)=sigma(n) then print(n); fi; od; end:
A015865 (10^10); # Paolo P. Lava, Mar 25 2013
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MATHEMATICA
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Select[Range[215000], DivisorSigma[1, #]==DivisorSigma[1, # + 5] &] (* Vincenzo Librandi, Mar 10 2014 *)
Position[Partition[DivisorSigma[1, Range[215000]], 6, 1], _?(#[[1]] == #[[6]]&), 1, Heads->False]//Flatten (* Harvey P. Dale, Sep 18 2021 *)
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PROG
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(PARI) is(n)=sigma(n)==sigma(n+5) \\ Charles R Greathouse IV, Mar 09 2014
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CROSSREFS
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Cf. A000203, A002961, A007373, A015861, A015863, A015866, A015867, A015876, A015877, A015880, A015881, A015882, A015883, A181647.
Sequence in context: A275031 A094655 A327364 * A276271 A145506 A256160
Adjacent sequences: A015862 A015863 A015864 * A015866 A015867 A015868
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KEYWORD
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nonn
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AUTHOR
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Robert G. Wilson v
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EXTENSIONS
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Corrected and extended by T. D. Noe, Oct 31 2006
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STATUS
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approved
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