

A007921


Numbers that are not the difference of two primes.


19



7, 13, 19, 23, 25, 31, 33, 37, 43, 47, 49, 53, 55, 61, 63, 67, 73, 75, 79, 83, 85, 89, 91, 93, 97, 103, 109, 113, 115, 117, 119, 121, 123, 127, 131, 133, 139, 141, 143, 145, 151, 153, 157, 159, 163, 167, 169, 173, 175, 181, 183, 185, 187, 193
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OFFSET

1,1


COMMENTS

Conjecturally, odd numbers n such that n+2 is composite.
Is this the same as A068780(2n1)  1?  J. Stauduhar, Aug 23 2012
A092953(a(n)) = 0.  Reinhard Zumkeller, Nov 10 2012
It seems that the sequence contains the squares of all primes except 2 and 3.  Ivan N. Ianakiev, Aug 29 2013


REFERENCES

F. Smarandache, Properties of Numbers, 1972. (See Smarandache odd sieve.)


LINKS

T. D. Noe, Table of n, a(n) for n = 1..10000
C. Dumitrescu & V. Seleacu, editors, Some Notions and Questions in Number Theory, Vol. I.
F. Smarandache, Only Problems, Not Solutions!, 4th ed., 1993, Problem 94.
Index entries for primes, gaps between


PROG

(PARI) is(n)=n%2 && !isprime(n+2) \\ On Polignac's conjecture; Charles R Greathouse IV, Jun 28 2013
(Haskell)
a007921 n = a007921_list !! (n1)
a007921_list = filter ((== 0) . a010051' . (+ 2)) [1, 3 ..]
 Reinhard Zumkeller, Jul 03 2015
(Python)
from sympy import isprime
print [n for n in range(1, 200) if n%2 and not isprime(n + 2)] # Indranil Ghosh, Jun 15 2017, after Charles R Greathouse IV


CROSSREFS

Cf. A048859.
Complement of A030173. Cf. A001223.
Cf. also A005408, A010051.
Sequence in context: A274437 A031194 A121058 * A092409 A124095 A109369
Adjacent sequences: A007918 A007919 A007920 * A007922 A007923 A007924


KEYWORD

nonn,easy,nice


AUTHOR

R. Muller


STATUS

approved



