A123930 a(n) = smallest k > 1 that is the sum of the proper divisors (cf. A001065) of at least n different numbers.

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

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: A002078 A000372 A333445 * 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