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A236468 Number of ordered ways to write n = k + m with k > 0 and m > 0 such that p = phi(k) + phi(m)/2 - 1, p + 2 and prime(p) - 2 are all prime, where phi(.) is Euler's totient function. 5
0, 0, 0, 0, 0, 0, 0, 2, 2, 1, 3, 1, 1, 2, 2, 4, 0, 1, 2, 2, 1, 2, 1, 1, 2, 0, 3, 2, 2, 3, 4, 2, 1, 2, 5, 3, 4, 0, 6, 6, 1, 3, 1, 5, 4, 5, 2, 5, 1, 7, 1, 3, 2, 5, 1, 4, 1, 7, 0, 5, 4, 1, 8, 1, 5, 5, 1, 2, 5, 4, 4, 4, 4, 1, 5, 1, 7, 3, 3, 2, 2, 1, 8, 3, 3, 2, 2, 2, 6, 3, 7, 2, 6, 5, 1, 1, 5, 4, 9, 3 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,8
COMMENTS
Conjecture: a(n) > 0 for every n = 250, 251, ....
This implies that there are infinitely many twin prime pairs {p, p + 2} with {prime(p) - 2, prime(p)} also a twin prime pair. It is stronger than the twin prime conjecture.
LINKS
EXAMPLE
a(33) = 1 since 33 = 7 + 26 with phi(7) + phi(26)/2 - 1 = 11, 11 + 2 = 13 and prime(11) - 2 = 31 - 2 = 29 all prime.
a(278) = 1 since 278 = 61 + 217 with phi(61) + phi(217)/2 - 1 = 60 + 90 - 1 = 149, 149 + 2 = 151 and prime(149) - 2 = 859 - 2 = 857 all prime.
MATHEMATICA
p[n_]:=PrimeQ[n]&&PrimeQ[n+2]&&PrimeQ[Prime[n]-2]
f[n_, k_]:=EulerPhi[k]+EulerPhi[n-k]/2-1
a[n_]:=Sum[If[p[f[n, k]], 1, 0], {k, 1, n-3}]
Table[a[n], {n, 1, 100}]
CROSSREFS
Sequence in context: A056044 A236097 A239319 * A116685 A355522 A268190
KEYWORD
nonn
AUTHOR
Zhi-Wei Sun, Jan 26 2014
STATUS
approved

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Last modified March 28 15:38 EDT 2024. Contains 371254 sequences. (Running on oeis4.)