OFFSET
1,1
COMMENTS
It appears that this sequence has 118 terms, the last of which is 490. All even numbers from 492 to 10^7 are the sum of two untouchable numbers.
LINKS
Donovan Johnson, Table of n, a(n) for n = 1..118
Eric Weisstein's World of Mathematics, Untouchable Number.
Wikipedia, Untouchable number
EXAMPLE
90 is not a term because it is the sum of two untouchable numbers (2 + 88). 92 is a term because no two untouchable numbers sum to 92.
PROG
(PARI) mx=10000; v=vector(mx); w=vector(mx); un=vector(1212); for(i=669, 1229, v[prime(i)+1]=1); for(i=2, 24989857, al=sigma(i)-i; if(al<=mx, v[al]=1)); c=0; for(i=2, mx, if(v[i]==0, c++; un[c]=i)); for(i=1, 1212, for(j=i, 1212, s=un[i]+un[j]; if(s<=mx, w[s]=1, next(2)))); n=0; forstep(i=2, mx, 2, if(w[i]==0, n++; write("b209249.txt", n " " i)))
CROSSREFS
KEYWORD
nonn
AUTHOR
Donovan Johnson, Jan 13 2013
STATUS
approved