A209249 Even numbers that are not the sum of two untouchable numbers.

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

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