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 A080355 a(1)=1; thereafter, a(n+1) = a(n) + 2^(prime(n)-1). 20
 1, 3, 7, 23, 87, 1111, 5207, 70743, 332887, 4527191, 272962647, 1346704471, 70066181207, 1169577808983, 5567624320087, 75936368497751, 4579535995868247, 292809912147579991, 1445731416754426967, 75232707711592633431, 1255824328429003936855, 5978190811298649150551 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Original name: a(1)=1; for n>1, a(n) = a(n-1) + 2^(j-1), where j = prime(n-1) is position of n-th 1 in A080339. Or, take an initial segment of A080339, stopping at the n-th 1, reverse and interpret as a binary number. E.g., to get the 4th term: 11101 -> 10111 = 23, so a(4) = 23. Indices of noncomposite terms in the sequence are 1, 2, 3, 4, 9, 310, 418, .... Next term (i.e., index of a prime), if it exists, is > 2000. See also post to SeqFan list by Tomasz Ordowski. - M. F. Hasler, Oct 30 2018 LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..475 T. Ordowski, Primes in primes, SeqFan list, Oct. 28, 2018 FORMULA a(n) = 1 + Sum_{k=1..n-1} 2^(prime(k)-1). MAPLE a:=n->1+add(2^(ithprime(k)-1), k=1..n-1): seq(a(n), n=1..25); # Muniru A Asiru, Oct 31 2018 MATHEMATICA RecurrenceTable[{a[1]==1, a[n] == 2^(Prime[n-1] - 1) + a[n-1]}, a, {n, 25}] (* Vincenzo Librandi, Oct 31 2018 *) nxt[{n_, a_}]:={n+1, a+2^(Prime[n]-1)}; NestList[nxt, {1, 1}, 30][[All, 2]] (* Harvey P. Dale, Aug 07 2019 *) PROG (PARI) apply( A080355(n)=1+sum(i=1, n-1, 2^(prime(i)-1)), [1..50]) \\ M. F. Hasler, Oct 30 2018 (MAGMA) [n le 1 select 1 else Self(n-1) + 2^(NthPrime(n-1)-1): n in [1..25]]; // Vincenzo Librandi, Oct 31 2018 CROSSREFS Cf. A076793. Sequence in context: A099152 A289317 A113860 * A100964 A080077 A096318 Adjacent sequences:  A080352 A080353 A080354 * A080356 A080357 A080358 KEYWORD nonn,easy,changed AUTHOR N. J. A. Sloane, based on information supplied by Artur Jasinski, Mar 21 2003 EXTENSIONS More terms from Vladeta Jovovic, Mar 26 2003 STATUS approved

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Last modified August 20 20:50 EDT 2019. Contains 326155 sequences. (Running on oeis4.)