A175220 The fifth nonprimes after the primes.

10, 10, 12, 14, 18, 20, 24, 25, 28, 35, 36, 44, 48, 49, 52, 58, 65, 66, 74, 77, 78, 85, 88, 94, 104, 108, 110, 114, 115, 118, 133, 136, 143, 144, 155, 156, 162, 169, 172, 178, 185, 186, 198, 200, 203, 204, 216, 230, 234, 235, 238, 245, 246, 256, 262, 268, 275

Offset

1,1

Comments

From Robert Israel, Jul 05 2016: (Start)

For n>1, there are the following cases:

If prime(n)+2 and prime(n)+4 are composite, then a(n) = prime(n)+5.

If exactly one of prime(n)+2 and prime(n)+4 is prime, and prime(n)+6 is composite, then a(n) = prime(n) + 6.

Otherwise, a(n) = prime(n) + 7. (End)

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Maple

N:= 1000: # to get all entries <= N
Primes:= select(isprime, [2, seq(i, i=3..N+7, 2)]):
nprimes:= nops(Primes):
A[1]:= 10:
A[2]:= 10:
for i from 3 to nprimes-1 do
  p:= Primes[i];
  if p + 5 > N then break fi;
  if Primes[i+1] > p + 4 then A[i]:= p + 5
  elif (i = nprimes-1 or Primes[i+2] <> p+6) and p+6 <= N  then A[i]:= p + 6
  elif p+7 <= N then A[i]:= p + 7
  else break
  fi
od:
seq(A[j], j=1..i-1); # Robert Israel, Jul 05 2016

Crossrefs

Sequence in context: A354049 A354113 A354114 * A272479 A167427 A227437

Adjacent sequences: A175217 A175218 A175219 * A175221 A175222 A175223

Keyword

nonn

Author

Jaroslav Krizek, Mar 06 2010

Status

approved