A261963 Smallest number that can be written as the sum of a Sophie Germain prime (A005384) and a practical number (A005153) in exactly n ways.

2, 3, 6, 23, 31, 35, 65, 59, 95, 131, 173, 233, 203, 239, 257, 299, 317, 323, 473, 443, 635, 563, 671, 719, 809, 701, 779, 839, 803, 1109, 971, 1049, 1139, 1343, 1103, 1409, 1433, 1607, 1481, 1499, 1559, 1523, 1769, 1679, 1643, 2069, 2063, 2309, 2111, 2141

Offset

0,1

Links

Table of n, a(n) for n=0..49.

Example

23 can be written as the sum of a Sophie Germain prime and a practical number in the following three ways: 3 + 20, 5 + 18, 11 + 12.
Since 23 is the smallest number that can be expressed like that in exactly three ways, a(3) = 23.

Prog

(PARI) \\ First define the function is_a005153(n) as in A005153
is_a005384(n) = ispseudoprime(n) && ispseudoprime(2*n+1)
count(n) = x=1; y=n-1; i=0; while(y > n/2, if((is_a005153(x) && is_a005384(y)) || (is_a005153(y) && is_a005384(x)), i++); x++; y--); i
a(n) = k=2; while(count(k)!=n, k++); k

Crossrefs

Cf. A005153, A005384, A209253.

Sequence in context: A261014 A000616 A233217 * A183179 A018300 A317973

Adjacent sequences: A261960 A261961 A261962 * A261964 A261965 A261966

Keyword

nonn

Author

Felix Fröhlich, Sep 06 2015

Status

approved