login
A083414
Write the numbers from 1 to n^2 consecutively in n rows of length n; let c(k) = number of primes in k-th column; a(n) = minimal c(k) for gcd(k,n) = 1.
6
0, 1, 1, 2, 1, 4, 1, 2, 3, 5, 2, 6, 1, 5, 5, 5, 2, 10, 2, 6, 5, 8, 3, 9, 5, 8, 5, 9, 4, 17, 3, 9, 7, 9, 6, 15, 4, 9, 8, 13, 4, 21, 3, 11, 10, 11, 4, 17, 5, 15, 9, 14, 5, 20, 8, 14, 9, 14, 6, 27, 6, 15, 12, 14, 9, 26, 6, 15, 12, 23, 5, 25, 3, 15, 13, 17, 8, 29, 7, 20, 12, 17, 7, 32
OFFSET
1,4
COMMENTS
Conjectured to be always positive for n>1.
Note that a(n) is large when phi(n), the number of integers relatively prime to n, is small and vice versa. - T. D. Noe, Jun 10 2003
The conjecture is true for all n <= 40000.
REFERENCES
See A083382 for references and links.
EXAMPLE
For n = 4 the array is
. 1 2 3 4
. 5 6 7 8
. 9 10 11 12
. 13 14 15 16
in which columns 1 and 3 contain 2 and 3 primes; therefore a(4) = 2.
MATHEMATICA
Table[minP=n; Do[If[GCD[c, n]==1, s=0; Do[If[PrimeQ[c+(r-1)*n], s++ ], {r, n}]; minP=Min[s, minP]], {c, n}]; minP, {n, 100}]
PROG
(Haskell)
a083414 n = minimum $ map c $ filter ((== 1) . (gcd n)) [1..n] where
c k = sum $ map a010051 $ enumFromThenTo k (k + n) (n ^ 2)
-- Reinhard Zumkeller, Jun 10 2012
CROSSREFS
Cf. A083415 and A083382 for primes in rows.
A084927 generalizes this to three dimensions.
Cf. A010051.
Sequence in context: A072064 A105498 A179289 * A334890 A171174 A171173
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Jun 10 2003
EXTENSIONS
More terms from Vladeta Jovovic and T. D. Noe, Jun 10 2003
STATUS
approved