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A159267 Largest m<=n such that 2^n+3^m is prime, 0 if no such m exists. 2
1, 2, 2, 4, 4, 5, 6, 5, 7, 9, 8, 11, 12, 9, 1, 15, 15, 17, 10, 11, 19, 18, 22, 17, 0, 21, 13, 20, 26, 26, 16, 27, 32, 33, 29, 35, 36, 30, 29, 33, 16, 37, 32, 28, 31, 38, 46, 47, 39, 34, 49, 43, 44, 26, 18, 53, 32, 39, 46, 59, 56, 58, 40, 63, 47, 49, 57, 33, 10, 26, 66, 61, 55, 22 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
Note: a(n)=0 means either there is no such m, or 2^n+1 is a Fermat prime, A019434. However, no such prime is known for n>16 and a(n)>0 for all n < 25. Thus a(n)=0 will practically always mean that there's no such m.
LINKS
MATHEMATICA
lmln[n_]:=Module[{m=n, c=2^n}, While[!PrimeQ[c+ 3^m]&&m>0, m--]; m]; Array[lmln, 80] (* Harvey P. Dale, Apr 05 2023 *)
PROG
(PARI) A159267(n)=forstep(m=n, 1, -1, ispseudoprime(2^n+3^m) & return(m))
CROSSREFS
Sequence in context: A288774 A113474 A089413 * A127311 A302929 A129229
KEYWORD
nonn
AUTHOR
M. F. Hasler, Apr 07 2009
STATUS
approved

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Last modified March 28 12:26 EDT 2024. Contains 371254 sequences. (Running on oeis4.)