A348335 - OEIS

A348335

a(n) = smallest k such that the sum of the divisors of the n numbers from k to k+n-1 equals sigma(k+n), or -1 if no such k exists.

14, 1, 591357

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OFFSET

1,1

COMMENTS

a(4) > 10^9, if it exists. - Amiram Eldar, Oct 13 2021

LINKS

Table of n, a(n) for n=1..3.

EXAMPLE

a(1) = 14 because sigma(14) = sigma(15) = 24; a(1) = A002961(1).

a(2) = 1 because sigma(1) + sigma(2) = 1 + 3 = 4, the same as sigma(3) = 4; a(2) = A104149(1).

a(3) = 591357 because sigma(591357) + sigma(591358) + sigma(591359) = 866880 + 890352 + 599760 = 2356992, the same as sigma(591360) = 2356992.

MATHEMATICA

a[n_] := Module[{sig = DivisorSigma[1, Range[n]], k = n + 1}, While[(s = DivisorSigma[1, k]) != Plus @@ sig, sig = Join[Drop[sig, 1], {s}]; k++]; k - n]; Array[a, 3] (* Amiram Eldar, Oct 29 2021 *)

PROG

(PARI) isok(m, nb) = sum(i=1, nb, sigma(m+i-1)) == sigma(m+nb);

a(n) = my(k=1); while (!isok(k, n), k++); k; \\ Michel Marcus, Oct 28 2021

CROSSREFS

Cf. A000203, A002961, A065900, A092403, A104149, A252922.

Sequence in context: A147716 A223516 A350888 * A372262 A040192 A112647

Adjacent sequences: A348332 A348333 A348334 * A348336 A348337 A348338

KEYWORD

nonn,bref,more

AUTHOR

Metin Sariyar, Oct 13 2021

STATUS

approved