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A226859 Number of prime sums in the process described in A226770. 2
1, 1, 1, 1, 1, 1, 2, 1, 3, 1, 3, 1, 3, 2, 3, 1, 4, 1, 4, 2, 5, 1, 5, 1, 6, 3, 7, 1, 6, 1, 7, 4, 7, 3, 8, 1, 9, 4, 9, 1, 9, 1, 9, 4, 10, 1, 9, 2, 10, 2, 11, 1, 11, 2, 13, 5, 14, 1, 13, 1, 12, 5, 12, 5, 13, 1, 13, 6, 14, 1, 14, 1, 13, 6, 14, 7, 15, 1, 15, 3, 15 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,7

LINKS

Peter J. C. Moses, Table of n, a(n) for n = 1..2000

FORMULA

a(n) = 1 iff either n = 5 or n + 1 = p or n + 1 = q^2, where p,q and q^2+q-1 are primes.

EXAMPLE

Let n=76. We have 77; d=7,11; 76+7=83 (prime), 76+11=87; d=3,29; 76+3=79(prime), 76+29=105; d=5,15,21,35; 76+5=81, 76+15=91, 76+21=97(prime), 76+35=111; d=9,27,13,37, 76+9=85,76+27=103(prime),76+13=89(prime), 76+37=113(prime), d=17, 76+17=93; d=31, 76+31=107(prime). Thus the set of prime sums is {83,79,97,103,89,113,107} and therefore a(76)=7.

MATHEMATICA

Table[(div=Most[Divisors[n+1]]; Count[n+FixedPoint[Union[Flatten[AppendTo[div, Map[Most[Divisors[n+#]]&, #]]]]&, div], _?PrimeQ]), {n, 50}] (* Peter J. C. Moses, Jun 20 2013 *)

CROSSREFS

Cf. A226770, A226856, A053184.

Sequence in context: A029236 A152188 A239930 * A025820 A109704 A073407

Adjacent sequences:  A226856 A226857 A226858 * A226860 A226861 A226862

KEYWORD

nonn

AUTHOR

Vladimir Shevelev, Jun 20 2013

EXTENSIONS

More terms from Peter J. C. Moses, Jun 20 2013

STATUS

approved

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Last modified March 8 13:25 EST 2021. Contains 341948 sequences. (Running on oeis4.)