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A189825 Least number k such that d(k-1) + d(k+1) = n, where d(k) is the number of divisors of k. 2
2, 4, 3, 14, 5, 7, 15, 11, 17, 19, 35, 29, 65, 41, 101, 79, 143, 71, 197, 161, 323, 169, 2917, 181, 577, 239, 575, 629, 899, 419, 1297, 701, 901, 721, 25599, 881, 5183, 1121, 9215, 2351, 4901, 1079, 107585, 1681, 36863, 2159, 3601, 2881, 11663, 2519 (list; graph; refs; listen; history; text; internal format)
OFFSET

3,1

COMMENTS

The function d(k-1) + d(k+1) is a measure of the compositeness of the numbers next to k. There is no k for n=1 and n=2. Some terms can be quite large; for example, a(99) = 6533135.

LINKS

Donovan Johnson, Table of n, a(n) for n = 3..880

FORMULA

Least k such that A189827(k) = n.

MATHEMATICA

nn = 100; t = Table[-1, {nn}]; t[[1]] = t[[2]] = 0; cnt = 2; n = 1; While[cnt < nn, n++; s = DivisorSigma[0, n-1] + DivisorSigma[0, n+1]; If[s <= nn && t[[s]] == -1, t[[s]] = n; cnt++]]; Drop[t, 2]

CROSSREFS

Cf. A175144.

Sequence in context: A200715 A297901 A079308 * A271878 A259476 A271363

Adjacent sequences:  A189822 A189823 A189824 * A189826 A189827 A189828

KEYWORD

nonn

AUTHOR

T. D. Noe, Apr 28 2011

STATUS

approved

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Last modified December 13 05:16 EST 2018. Contains 318082 sequences. (Running on oeis4.)