The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 56th year, we are closing in on 350,000 sequences, and we’ve crossed 9,700 citations (which often say “discovered thanks to the OEIS”).

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A220355 Numbers n such that n + n# / n is prime, where n# = product of all primes <= n. 1
 1, 2, 3, 5, 6, 7, 10, 15, 17, 19, 23, 34, 38, 39, 46, 47, 57, 59, 61, 78, 79, 177, 203, 209, 214, 249, 354, 371, 373, 501, 663, 755, 890, 922, 986, 1015, 1041, 1118, 1383, 1687, 1802, 1994, 2042, 2567, 2778, 3239, 3387, 3445, 3462, 3466, 4206, 4322, 4837, 5811 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Squarefree A005117 numbers n such that n + A034386(n)/n is prime. LINKS Michael B. Porter, Table of n, a(n) for n = 1..60 with data from Thomas Ordowski, Marek Wolf, and Joerg Arndt; contains all terms <= 10^4. EXAMPLE 6 is in the sequence because 6 + 6#/6 = 6 + 5#/6 = 6 + 30/6 = 11 is prime. 4 is not in the sequence because 4 + 4#/4 = 4 + 6/4 = 11/2 is not an integer. 11 is not in the sequence because 11 + 11#/11 = 11 + 7# = 11 + 210 = 221 is not prime (221 = 13 * 17). PROG (PARI) pp(n) = { my(r=1); forprime(p=2, n, r*=p); r; } /* we need to take care about n+pp(n)/n not always being integral: */ is(n) = { my(r=n+pp(n)/n); return( (type(r)==type(1)) && ispseudoprime(r) ); } for (n=1, 10^4, if ( is(n), print1(n, ", ") ) ); /* Joerg Arndt, Apr 14 2013 */ CROSSREFS Cf. A005117, A034386. Sequence in context: A194359 A345166 A277006 * A059041 A129128 A164922 Adjacent sequences:  A220352 A220353 A220354 * A220356 A220357 A220358 KEYWORD nonn AUTHOR Thomas Ordowski, Apr 13 2013 EXTENSIONS a(22)-a(35) from Marek Wolf, Apr 14 2013 a(36)-a(54) from Joerg Arndt, Apr 14 2013 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified December 8 22:49 EST 2021. Contains 349596 sequences. (Running on oeis4.)