A296375 Least x such that sigma(x) = prime(n)+1, where sigma = A000203.

2, 3, 5, 7, 6, 13, 10, 19, 14, 29, 21, 37, 20, 43, 33, 34, 24, 61, 67, 30, 73, 57, 44, 40, 52, 101, 63, 85, 109, 74, 93, 86, 137, 76, 149, 111, 157, 163, 60, 173, 88, 117, 105, 193, 197, 199, 211, 84, 147, 229, 90, 114, 241, 96, 257, 215, 136, 201, 277, 281, 283, 164, 172, 126, 313, 317, 331, 337

Offset: 1

Author

Robert Israel, Dec 27 2017

Keywords

• nonn

Comments

a(n) <= prime(n).

Examples

			For n=5, prime(5)+1 = 12; sigma(x) = 12 for x=6 and x=11, so a(5) = 6.
		

Links

• Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A066077.

Programs

Maple

N:= 100: # to get a(1)..a(N)
for i from 1 to N do T(ithprime(i)):= i od:
count:= 0:
for n from 1 while count < N do
v:= numtheory:-sigma(n);
if T(v)::posint then
  A[T(v)]:= n;
  T(v):= 0;
    count:= count+1;
  fi
od:
seq(A[i],i=1..N);

PARI

first(n) = my(res = vector(n), p = 2); for(k=1, n, for(x=2, p, if(sigma(x) == p+1, res[k] = x; break())); p = nextprime(p+1)); res \\ Iain Fox, Dec 28 2017

PARI

first(n)=my(v=vector(n),P=primes(n),left=n,t); forfactored(x=2,P[n], t=setsearch(P,sigma(x)-1); if(t && v[t]==0, v[t]=x[1]; if(left--==0, return(v)))) \\ Charles R Greathouse IV, Dec 28 2017