This site is supported by donations to The OEIS Foundation.

 Please make a donation to keep the OEIS running. We are now in our 55th year. In the past year we added 12000 new sequences and reached 8000 citations (which often say "discovered thanks to the OEIS"). We need to raise money to hire someone to manage submissions, which would reduce the load on our editors and speed up editing. Other ways to donate

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A195268 Numbers such that the sum of the odd divisors is prime. 4
 9, 18, 25, 36, 50, 72, 100, 144, 200, 288, 289, 400, 576, 578, 729, 800, 1152, 1156, 1458, 1600, 1681, 2304, 2312, 2401, 2916, 3200, 3362, 3481, 4608, 4624, 4802, 5041, 5832, 6400, 6724, 6962, 7921, 9216, 9248, 9604, 10082, 10201, 11664, 12800 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Odd numbers n^2 such that sigma(n^2) is prime, times an arbitrary power of two. [Charles R Greathouse IV, Sep 14 2011] LINKS Charles R Greathouse IV, Table of n, a(n) for n = 1..10000 EXAMPLE The divisors of 2312 are  { 1, 2, 4, 8, 17, 34, 68, 136, 289, 578, 1156, 2312}, and the sum of the odd divisors 1 + 17 + 289 =307 is prime. Hence 2312 = 2*34^2 is in the sequence. MAPLE with(numtheory):for n from 1 to 20000 do:x:=divisors(n):n1:=nops(x):s:=0:for m from 1 to n1 do:if irem(x[m], 2)=1 then s:=s+x[m]:fi:od:if type(s, prime)=true  then printf(`%d, `, n): else fi:od: PROG (PARI) list(lim)=my(v=List(), t); forstep(k=3, sqrt(lim), 2, if(isprime(sigma(t=k^2)), listput(v, t); while((t<<=1)<=lim, listput(v, t)))); vecsort(Vec(v)) \\ Charles R Greathouse IV, Sep 14 2011 CROSSREFS Subsequence of A028982. Sequence in context: A034046 A069562 A072502 * A328252 A227279 A102042 Adjacent sequences:  A195265 A195266 A195267 * A195269 A195270 A195271 KEYWORD nonn AUTHOR Michel Lagneau, Sep 14 2011 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 5 10:45 EST 2019. Contains 329751 sequences. (Running on oeis4.)