

A209249


Even numbers that are not the sum of two untouchable numbers.


1



2, 6, 8, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 56, 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78, 80, 82, 84, 86, 88, 92, 94, 96, 100, 102, 106, 108, 110, 112, 114, 116, 118, 120, 124, 128, 130, 132, 134, 136, 138
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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

Cf. A005114.
Sequence in context: A064212 A056906 A257056 * A047238 A189933 A229488
Adjacent sequences: A209246 A209247 A209248 * A209250 A209251 A209252


KEYWORD

nonn


AUTHOR

Donovan Johnson, Jan 13 2013


STATUS

approved



