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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

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