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 A109926 Least k such that k-2^r is prime for n values of r. Index of the first occurrence of n in A109925. 1
 1, 3, 4, 15, 21, 45, 75, 465, 1095, 2145, 4935, 14955, 80685, 229845, 1295325, 1575285, 9700575, 20435415, 15054105, 53999715, 2282745465 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS It appears that 3 and 5 divide a(n) for n>4. Note that a(18) 2. Then n-2^k is not == 0 (mod 3) and a prime is more probable. - Robert G. Wilson v, Jul 21 2005 Conjecture: a(n+15)==0 (mod 30) for n > 4. - Robert G. Wilson v, Jul 21 2005 a(n) > 10^10 for n >= 21. - Donovan Johnson, Jan 21 2009 LINKS EXAMPLE a(4) = 21, 21-2 =19, 21-4 = 17, 21-8 = 13, 21-16 = 5, 21 is the smallest number that gives four such primes. MATHEMATICA t=Table[cnt=0; r=1; While[r

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Last modified April 13 20:46 EDT 2021. Contains 342941 sequences. (Running on oeis4.)