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A123930 a(n) = smallest k > 1 that is the sum of the proper divisors (cf. A001065) of at least n different numbers. 2
2, 3, 6, 21, 31, 31, 49, 73, 73, 91, 115, 121, 121, 121, 169, 169, 211, 211, 211, 211, 211, 301, 331, 331, 331, 361, 391, 391, 421, 421, 421, 421, 421, 511, 511, 631, 631, 631, 631, 631, 631, 631, 721, 721, 721, 781, 781, 841, 841, 841, 841, 841, 841, 841, 841 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

LINKS

Donovan Johnson, Table of n, a(n) for n = 0..1000

Eric Weisstein's World of Mathematics, Sum of Proper Divisors Function.

EXAMPLE

a(0)=2 because 2 is the smallest number that is not the sum of proper divisors of any number (untouchable). a(6)=49 since 49 is the smallest number that is the sum of proper divisors of 6 different numbers (75, 215, 287, 407, 527, 551).

PROG

(PARI) {z=850; m=200000; v=vector(z); for(n=2, m, s=sigma(n)-n; if(s<z, v[s]++)); r=0; for(j=2, z, while(r<=v[j], r=r+1; print1(j, ", ")))} \\ Klaus Brockhaus, Nov 27 2006

CROSSREFS

Cf. A001065, A070015, A048138, A125601.

Sequence in context: A277876 A002078 A000372 * A238895 A125601 A292811

Adjacent sequences:  A123927 A123928 A123929 * A123931 A123932 A123933

KEYWORD

nonn

AUTHOR

Sergio Pimentel, Nov 22 2006

EXTENSIONS

Edited and extended by Klaus Brockhaus, Nov 27 2006

STATUS

approved

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Last modified January 18 04:43 EST 2019. Contains 319265 sequences. (Running on oeis4.)