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

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A344366 Integers k such that the sum of squares of digits of both k and k-1 are prime. 0
 12, 102, 111, 120, 160, 230, 250, 380, 410, 450, 520, 560, 720, 780, 830, 870, 1002, 1011, 1020, 1060, 1100, 1101, 1110, 1370, 1640, 1680, 1910, 1950, 1970, 1990, 2030, 2050, 2340, 2670, 2920, 3080, 3170, 3240, 3420, 3460, 3550, 3570, 3710, 3840, 3860, 4010 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Integers k such that k and k-1 are both in A108662. Terms are never prime. They cannot end in the digits 3,4,5,6,7,8,9. If k is a term, phi(k) is divisible by 4. The set of such numbers is infinite. LINKS Charles U. Lonappan, Consecutive natural numbers whose sum of squares of digits is prime, International Journal of Research in Engineering and Science (IJRES), Volume 9 Issue 1 (2021) pp. 14-16. EXAMPLE 12 is in the sequence because the sum of squares of digits of 12 is 5 and that of 11 is 2, and both 5 and 2 are prime numbers. MATHEMATICA q[n_] := PrimeQ[Plus @@ (IntegerDigits[n]^2)]; Select[Range[2, 5000], q[#-1] && q[#] &] (* Amiram Eldar, May 19 2021 *) PROG (PARI) isok(k) = isprime(norml2(digits(k-1))) && isprime(norml2(digits(k))); \\ Michel Marcus, May 24 2021 CROSSREFS Cf. A108662. Sequence in context: A008547 A109020 A099299 * A304504 A344279 A022736 Adjacent sequences:  A344363 A344364 A344365 * A344367 A344368 A344369 KEYWORD nonn,base AUTHOR Charles U. Lonappan, May 19 2021 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 July 24 20:26 EDT 2021. Contains 346273 sequences. (Running on oeis4.)