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A097523
a(n) = least k such that k - prime(n) and k + prime(n) are both prime.
1
5, 8, 8, 10, 18, 16, 20, 22, 30, 32, 36, 42, 48, 46, 50, 56, 72, 66, 70, 78, 76, 84, 90, 92, 100, 132, 108, 120, 114, 116, 130, 138, 140, 142, 162, 156, 160, 168, 170, 176, 210, 186, 198, 196, 200, 202, 222, 226, 230, 232, 246, 252, 246, 258, 264, 294, 272, 276
OFFSET
1,1
FORMULA
a(n) = A087711(A000040(n)). - Robert Israel, Jul 26 2015
EXAMPLE
Prime(10) = 29; both 32 - 29 = 3 and 32 + 29 = 61 are prime, and 32 is the smallest integer for which this is the case, so a(10) = 32.
MAPLE
f:= proc(n) local p, k;
p:= ithprime(n);
for k from p+1 by 2 do
if isprime(k+p) and isprime(k-p) then return k fi
od
end proc:
f(1):= 5:
map(f, [$1..100]); # Robert Israel, Jul 26 2015
MATHEMATICA
f[n_] := Block[{k = Prime[n], p = Prime[n]}, While[ !PrimeQ[k - p] || !PrimeQ[k + p], k++ ]; k]; Table[ f[n], {n, 60}] (* Robert G. Wilson v, Aug 28 2004 *)
CROSSREFS
Sequence in context: A198606 A031165 A113729 * A250123 A335298 A251687
KEYWORD
easy,nonn
AUTHOR
Pierre CAMI, Aug 27 2004
EXTENSIONS
Corrected by Robert G. Wilson v, Aug 28 2004
STATUS
approved