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 A252021 Primes p such that (p - 1)/tau(p - 1) is also prime. 2
 13, 19, 41, 61, 89, 137, 157, 229, 233, 277, 349, 373, 569, 709, 733, 809, 853, 857, 877, 997, 1049, 1069, 1097, 1193, 1213, 1237, 1433, 1669, 1789, 1913, 2153, 2293, 2389, 2677, 2749, 2777, 2797, 3209, 3229, 3253, 3373, 3449, 3517, 3593, 3733, 3833, 3929, 4073, 4457, 4549, 4597, 4793, 4813, 4909, 4937, 5197, 5273 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Amiram Eldar, Table of n, a(n) for n = 1..10000 EXAMPLE a(1) = 13, since 12/tau(12) = 2. a(2) = 19, since 18/tau(18) = 3. a(4) = 61, since 60/tau(60) = 5. MATHEMATICA Select[Prime[Range[1000]], PrimeQ[(# - 1)/DivisorSigma[0, # - 1]] &] PROG (MAGMA) [p:p in PrimesUpTo(5300)| ((p-1) mod NumberOfDivisors(p-1) eq 0) and IsPrime((p-1) div NumberOfDivisors(p-1)) ]; // Marius A. Burtea, Dec 30 2019 CROSSREFS Cf. A000005, A008328, A033950. Sequence in context: A000922 A107188 A029478 * A216101 A096455 A124199 Adjacent sequences:  A252018 A252019 A252020 * A252022 A252023 A252024 KEYWORD nonn,easy AUTHOR Carlos Eduardo Olivieri, Mar 21 2015 STATUS approved

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Last modified February 21 02:02 EST 2020. Contains 332086 sequences. (Running on oeis4.)