A122706 Smallest prime p such that p^n is equal to the sum of 3 consecutive primes.
23, 7, 11, 29, 79, 29, 509, 53, 467, 1571, 61, 7, 1553, 31, 1097, 11, 397, 11, 163, 677, 23, 103, 1723, 11, 1759, 67, 433, 149, 919, 2879, 293, 9907, 1103, 1153, 179, 6199, 2683, 1877, 4373, 4679, 953, 2341, 8069, 3779, 3691, 28463, 991, 1061, 2447, 5471
Offset: 1
Keywords
Examples
a(1) = 23 because A070934(1) = p(3) + p(4) + p(5) = 5 + 7 + 11 = 23 is prime, but p(1) + p(2) + p(3) = 2 + 3 + 5 = 10 is composite and p(2) + p(3) + p(4) = 3 + 5 + 7 = 15 is composite. a(2) = 7 because A122654(1) = 7 is prime and p(6) + p(7) + p(8) = 13 + 17 + 19 = 49 = 7^2, but p(k) + p(k+1) + p(k+2) are not squares for 0 < k < 6.
Links
- Robert G. Wilson v, Table of n, a(n) for n = 1..100
Programs
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Mathematica
PrevPrim[n_] := Block[{k = n - 1}, While[ !PrimeQ[k], k-- ]; k]; NextPrim[n_] := Block[{k = n + 1}, While[ !PrimeQ[k], k++ ]; k]; f[n_] := Block[{p = If[n < 2, 5, 3]}, While[r = PrevPrim@ Floor[p^n/3]; q = PrevPrim@r; s = NextPrim@r; t = NextPrim@s; p^n != q + r + s && p^n != r + s + t, p = NextPrim@p]; p]; Array[f, 50] (* Robert G. Wilson v *)
Formula
For m = (p^n)/3 (not an integer), if q,r are largest primes and s,t are smallest primes such that q < r < m < s < t, then p^n must equal either q+r+s or r+s+t. - Robert G. Wilson v
Extensions
a(5)-a(50) from Robert G. Wilson v, Sep 26 2006
Comments