OFFSET
1,1
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
Listing the first eight terms of A001358 gives us:
n: 1, 2, 3, 4, 5, 6, 7, 8, ...
4, 6, 9, 10, 14, 15, 21, 22, ...
We see that 4 can be represented as 6-2, 6 can be represented as 4+2 or 9-3 or 10-4, 9 can be represented as 14-5 or 15-6, but 10 cannot be represented by any such sum or difference as 4+1, 6+2, 9+3, 14-5, 15-6, 21-7, and also any difference A001358(n)-n after that will miss it. Thus 10 is the first semiprime included in this sequence.
MAPLE
N:= 2000: # to use semiprimes <= N
Primes:= select(isprime, [2, seq(i, i=3..N, 2)]):
Semiprimes:= select(`<=`, {seq(seq(Primes[i]*Primes[j], i=1..j), j=1..nops(Primes))}, N):
sort(convert(Semiprimes minus {seq}(i+Semiprimes[i], i=1..nops(Semiprimes)) minus {seq}(Semiprimes[i]-i, i=1..nops(Semiprimes))), list)); # Robert Israel, Dec 20 2015
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Juri-Stepan Gerasimov, Nov 22 2010
EXTENSIONS
Corrected by D. S. McNeil, Nov 23 2010
Name clarified and Example section added by Antti Karttunen, Dec 20 2015
STATUS
approved