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 A181603 Twin primes ending in 1. 7
 11, 31, 41, 61, 71, 101, 151, 181, 191, 241, 271, 281, 311, 421, 431, 461, 521, 571, 601, 641, 661, 811, 821, 881, 1021, 1031, 1051, 1061, 1091, 1151, 1231, 1291, 1301, 1321, 1451, 1481, 1621, 1721, 1871, 1931, 1951, 2081, 2111, 2131, 2141, 2311, 2341, 2381 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS These are twin primes == 1 (mod 30) or == 11 (mod 30) or == 21 (mod 30). In the first case they cannot be lower twin primes because the upper ones would be == 3 (mod 30) and divisible by 3. In the second case they cannot be upper twin primes because the lower ones would be == 9 (mod 30) and divisible by 3. The last case is excluded because that implies they are divisible by 3. In summary the upper twin primes in here are given by A282326, the lower twin primes in here by A282321. - R. J. Mathar, Feb 14 2017 LINKS Harvey P. Dale, Table of n, a(n) for n = 1..1000 Omar E. Pol, Determinacion geometrica de los numeros primos y perfectos MAPLE isA181603 := proc(p) if isprime(p) and (isprime(p-2) or isprime(p+2)) then if modp(p, 10) = 1 then true; else false; end if ; else false; end if; end proc: for n from 1 to 1000 do if isA181603(n) then printf("%d, ", n) ; end if; end do: # R. J. Mathar, Feb 14 2017 MATHEMATICA Select[Prime@ Range@ 360, Mod[ #, 10] == 1 && (PrimeQ[ # - 2] || PrimeQ[ # + 2]) &] (* Robert G. Wilson v, Nov 06 2010 *) Select[Flatten[Select[Partition[Prime[Range[400]], 2, 1], #[[2]]-#[[1]] == 2&]], Mod[#, 10]==1&] (* Harvey P. Dale, Oct 24 2021 *) CROSSREFS Cf. A001097, A181604, A181605, A181606. Sequence in context: A040172 A040160 A040981 * A040983 A040979 A181182 Adjacent sequences: A181600 A181601 A181602 * A181604 A181605 A181606 KEYWORD base,nonn AUTHOR Omar E. Pol, Nov 01 2010 EXTENSIONS More terms from Robert G. Wilson v, Nov 06 2010 STATUS approved

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Last modified February 25 10:32 EST 2024. Contains 370321 sequences. (Running on oeis4.)