Problem:
Score : 500 points

Problem Statement

Consider sequences \{A_1,...,A_N\} of length N consisting of integers between 1 and K (inclusive).

There are K^N such sequences. Find the sum of \gcd(A_1, ..., A_N) over all of them.

Since this sum can be enormous, print the value modulo (10^9+7).

Here \gcd(A_1, ..., A_N) denotes the greatest common divisor of A_1, ..., A_N.

Constraints

2 \leq N \leq 10^5

1 \leq K \leq 10^5

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N K

Output

Print the sum of \gcd(A_1, ..., A_N) over all K^N sequences, modulo (10^9+7).

Sample Input 1

3 2

Sample Output 1

9

\gcd(1,1,1)+\gcd(1,1,2)+\gcd(1,2,1)+\gcd(1,2,2)
+\gcd(2,1,1)+\gcd(2,1,2)+\gcd(2,2,1)+\gcd(2,2,2)
=1+1+1+1+1+1+1+2=9

Thus, the answer is 9.

Sample Input 2

3 200

Sample Output 2

10813692

Sample Input 3

100000 100000

Sample Output 3

742202979

Be sure to print the sum modulo (10^9+7).

Solution in Common Lisp (SBCL 2.0.3) (judge verdict: Accepted)

(eval-when (:compile-toplevel :load-toplevel :execute)
  (sb-int:defconstant-eqx OPT
    #+swank '(optimize (speed 3) (safety 2))
    #-swank '(optimize (speed 3) (safety 0) (debug 0))
    #'equal)
  #+swank (ql:quickload '(:cl-debug-print :fiveam) :silent t)
  #-swank (set-dispatch-macro-character
           ;; enclose the form with VALUES to avoid being captured by LOOP macro
           #\# #\> (lambda (s c p) (declare (ignore c p)) `(values ,(read s nil nil t)))))
#+swank (cl-syntax:use-syntax cl-debug-print:debug-print-syntax)
#-swank (disable-debugger) ; for CS Academy

;; BEGIN_INSERTED_CONTENTS
;; TODO: more efficient handling when modulus is (unsigned-byte 31) or
;; (unsigned-byte 32)
(declaim (inline mod-power))
(defun mod-power (base power modulus)
  "BASE := integer
POWER, MODULUS := non-negative fixnum"
  (declare ((integer 0 #.most-positive-fixnum) modulus power)
           (integer base))
  (labels ((recur (x p)
             (declare ((integer 0 #.most-positive-fixnum) x p)
                      #+sbcl (values (integer 0 #.most-positive-fixnum)))
             (cond ((zerop p) (mod 1 modulus))
                   ((evenp p) (recur (mod (* x x) modulus) (ash p -1)))
                   (t (mod (* x (recur x (- p 1))) modulus)))))
    (recur (mod base modulus) power)))

;;;
;;; Arithmetic operations with static modulus
;;;

;; FIXME: Currently MOD* and MOD+ doesn't apply MOD when the number of
;; parameters is one.
(defmacro define-mod-operations (divisor)
  `(progn
     (defun mod* (&rest args)
       (reduce (lambda (x y) (mod (* x y) ,divisor)) args))

     (defun mod+ (&rest args)
       (reduce (lambda (x y) (mod (+ x y) ,divisor)) args))

     #+sbcl
     (eval-when (:compile-toplevel :load-toplevel :execute)
       (locally (declare (muffle-conditions warning))
         (sb-c:define-source-transform mod* (&rest args)
           (if (null args)
               1
               (reduce (lambda (x y) `(mod (* ,x ,y) ,',divisor)) args)))
         (sb-c:define-source-transform mod+ (&rest args)
           (if (null args)
               0
               (reduce (lambda (x y) `(mod (+ ,x ,y) ,',divisor)) args)))))

     (define-modify-macro incfmod (delta)
       (lambda (x y) (mod (+ x y) ,divisor)))

     (define-modify-macro decfmod (delta)
       (lambda (x y) (mod (- x y) ,divisor)))

     (define-modify-macro mulfmod (multiplier)
       (lambda (x y) (mod (* x y) ,divisor)))))


;;;
;;; Fast Zeta/Moebius transforms w.r.t. divisor or multiple in O(nloglog(n)).
;;;

(declaim (inline divisor-transform!))
(defun divisor-transform! (vector &optional (op+ #'+) (handle-zero t))
  "Sets each VECTOR[i] to the sum of VECTOR[d] for all the divisors d of i in
O(nloglog(n)). Ignores VECTOR[0] when HANDLE-ZERO is NIL."
  (declare (vector vector))
  (let* ((n (length vector))
         (sieve (make-array n :element-type 'bit :initial-element 1)))
    (when handle-zero
      (loop for i from 1 below n
            do (setf (aref vector 0)
                     (funcall op+ (aref vector 0) (aref vector i)))))
    (loop for p from 2 below n
          when (= 1 (sbit sieve p))
          do (loop for k from 1 below (ceiling n p)
                   for pmult of-type fixnum = (* k p)
                   do (setf (sbit sieve pmult) 0)
                      (setf (aref vector pmult)
                            (funcall op+ (aref vector pmult) (aref vector k)))))
    vector))

(declaim (inline inverse-divisor-transform!))
(defun inverse-divisor-transform! (vector &optional (op- #'-) (handle-zero t))
  "Does the inverse transform of DIVISOR-TRANSFORM! in O(nloglog(n)). Ignores
VECTOR[0] when HANDLE-ZERO is NIL."
  (declare (vector vector))
  (let* ((n (length vector))
         (sieve (make-array n :element-type 'bit :initial-element 1)))
    (loop for p from 2 below n
          when (= 1 (sbit sieve p))
          do (loop for k from (- (ceiling n p) 1) downto 1
                   for pmult of-type fixnum = (* k p)
                   do (setf (sbit sieve pmult) 0)
                      (setf (aref vector pmult)
                            (funcall op- (aref vector pmult) (aref vector k)))))
    (when handle-zero
      (loop for i from 1 below n
            do (setf (aref vector 0)
                     (funcall op- (aref vector 0) (aref vector i)))))
    vector))

(declaim (inline multiple-transform!))
(defun multiple-transform! (vector &optional (op+ #'+) (handle-zero t))
  "Sets each VECTOR[i] to the sum of VECTOR[m] for all the multiples m of i in
O(nloglog(n)). (To be precise, all the multiples smaller than the length of
VECTOR.) Ignores VECTOR[0] when HANDLE-ZERO is NIL."
  (declare (vector vector))
  (let* ((n (length vector))
         (sieve (make-array n :element-type 'bit :initial-element 1)))
    (loop for p from 2 below n
          when (= 1 (sbit sieve p))
          do (loop for k from (- (ceiling n p) 1) downto 1
                   for pmult of-type fixnum = (* k p)
                   do (setf (sbit sieve pmult) 0)
                      (setf (aref vector k)
                            (funcall op+ (aref vector k) (aref vector pmult)))))
    (when handle-zero
      (loop for i from 1 below n
            do (setf (aref vector i)
                     (funcall op+ (aref vector 0) (aref vector i)))))
    vector))

(declaim (inline inverse-multiple-transform!))
(defun inverse-multiple-transform! (vector &optional (op- #'-) (handle-zero t))
  "Does the inverse transform of MULTIPLE-TRANSFORM!. Ignores VECTOR[0] when
HANDLE-ZERO is NIL."
  (declare (vector vector))
  (let* ((n (length vector))
         (sieve (make-array n :element-type 'bit :initial-element 1)))
    (when handle-zero
      (loop for i from 1 below n
            do (setf (aref vector i)
                     (funcall op- (aref vector i) (aref vector 0)))))
    (loop for p from 2 below n
          when (= 1 (sbit sieve p))
          do (loop for k from 1 below (ceiling n p)
                   for pmult of-type fixnum = (* k p)
                   do (setf (sbit sieve pmult) 0)
                      (setf (aref vector k)
                            (funcall op- (aref vector k) (aref vector pmult)))))
    vector))

;;;
;;; (Slower) Zeta/Moebius transforms w.r.t. divisor or multiple in O(nlog(n))
;;;

#|
(declaim (inline divisor-transform!))
(defun divisor-transform! (vector &optional (op+ #'+) (handle-zero t))
  "Sets each VECTOR[i] to the sum of VECTOR[d] for all the divisors d of i in
O(nlog(n))."
  (declare (vector vector))
  (let ((n (length vector)))
    (when handle-zero
      (loop for i from 1 below n
            do (setf (aref vector 0)
                     (funcall op+ (aref vector 0) (aref vector i)))))
    (loop for i from (- (ceiling n 2) 1) downto 1
          do (loop for j from (+ i i) below n by i
                   do (setf (aref vector j)
                            (funcall op+ (aref vector i) (aref vector j)))))
    vector))

(declaim (inline inverse-divisor-transform!))
(defun inverse-divisor-transform! (vector &optional (op- #'-) (handle-zero t))
  "Does the inverse transform of DIVISOR-TRANSFORM! in O(nlog(n))."
  (declare (vector vector))
  (let ((n (length vector)))
    (loop for i from 1 below (ceiling n 2)
          do (loop for j from (+ i i) below n by i
                   do (setf (aref vector j)
                            (funcall op- (aref vector j) (aref vector i)))))
    (when handle-zero
      (loop for i from 1 below n
            do (setf (aref vector 0)
                     (funcall op- (aref vector 0) (aref vector i)))))
    vector))


(declaim (inline multiple-transform!))
(defun multiple-transform! (vector &optional (op+ #'+) (handle-zero t))
  "Sets each VECTOR[i] to the sum of VECTOR[m] for all the multiples m of i in
O(nlog(n)). (To be precise, all the multiples smaller than the length of
VECTOR.)"
  (declare (vector vector))
  (let ((n (length vector)))
    (loop for i from 1 below (ceiling n 2)
          do (loop for j from (+ i i) below n by i
                   do (setf (aref vector i)
                            (funcall op+ (aref vector i) (aref vector j)))))
    (when handle-zero
      (loop for i from 1 below n
            do (setf (aref vector i)
                     (funcall op+ (aref vector 0) (aref vector i)))))
    vector))


(declaim (inline inverse-multiple-transform!))
(defun inverse-multiple-transform! (vector &optional (op- #'-) (handle-zero t))
  "Does the inverse transform of MULTIPLE-TRANSFORM! in O(nlog(n))."
  (declare (vector vector))
  (let ((n (length vector)))
    (when handle-zero
      (loop for i from 1 below n
            do (setf (aref vector i)
                     (funcall op- (aref vector i) (aref vector 0)))))
    (loop for i from (- (ceiling n 2) 1) downto 1
          do (loop for j from (+ i i) below n by i
                   do (setf (aref vector i)
                            (funcall op- (aref vector i) (aref vector j)))))
    vector))
;|#

(defmacro dbg (&rest forms)
  #+swank
  (if (= (length forms) 1)
      `(format *error-output* "~A => ~A~%" ',(car forms) ,(car forms))
      `(format *error-output* "~A => ~A~%" ',forms `(,,@forms)))
  #-swank (declare (ignore forms)))

(defmacro define-int-types (&rest bits)
  `(progn
     ,@(mapcar (lambda (b) `(deftype ,(intern (format nil "UINT~A" b)) () '(unsigned-byte ,b))) bits)
     ,@(mapcar (lambda (b) `(deftype ,(intern (format nil "INT~A" b)) () '(signed-byte ,b))) bits)))
(define-int-types 2 4 7 8 15 16 31 32 62 63 64)

(declaim (inline println))
(defun println (obj &optional (stream *standard-output*))
  (let ((*read-default-float-format* 'double-float))
    (prog1 (princ obj stream) (terpri stream))))

(defconstant +mod+ 1000000007)

;;;
;;; Body
;;;

(define-mod-operations +mod+)
(defun main ()
  (declare #.OPT)
  (let* ((n (read))
         (k (read))
         (dp (make-array (+ k 1) :element-type 'uint31 :initial-element 0)))
    (declare (uint31 n k))
    (loop for x from 1 to k
          do (setf (aref dp x) (mod-power (floor k x) n +mod+)))
    (inverse-multiple-transform! dp
                                 (lambda (x y)
                                   (declare (uint31 x y))
                                   (mod+ x (the uint31 (- +mod+ y))))
                                 nil)
    (let ((res 0))
      (declare (uint62 res))
      (loop for x from 1 to k
            do (incf res (mod* x (aref dp x))))
      (println (mod res +mod+)))))

#-swank (main)

;;;
;;; Test and benchmark
;;;

#+swank
(defun io-equal (in-string out-string &key (function #'main) (test #'equal))
  "Passes IN-STRING to *STANDARD-INPUT*, executes FUNCTION, and returns true if
the string output to *STANDARD-OUTPUT* is equal to OUT-STRING."
  (labels ((ensure-last-lf (s)
             (if (eql (uiop:last-char s) #\Linefeed)
                 s
                 (uiop:strcat s uiop:+lf+))))
    (funcall test
             (ensure-last-lf out-string)
             (with-output-to-string (out)
               (let ((*standard-output* out))
                 (with-input-from-string (*standard-input* (ensure-last-lf in-string))
                   (funcall function)))))))

#+swank
(defun get-clipbrd ()
  (with-output-to-string (out)
    (run-program "powershell.exe" '("-Command" "Get-Clipboard") :output out :search t)))

#+swank (defparameter *this-pathname* (uiop:current-lisp-file-pathname))
#+swank (defparameter *dat-pathname* (uiop:merge-pathnames* "test.dat" *this-pathname*))

#+swank
(defun run (&optional thing (out *standard-output*))
  "THING := null | string | symbol | pathname

null: run #'MAIN using the text on clipboard as input.
string: run #'MAIN using the string as input.
symbol: alias of FIVEAM:RUN!.
pathname: run #'MAIN using the text file as input."
  (let ((*standard-output* out))
    (etypecase thing
      (null
       (with-input-from-string (*standard-input* (delete #\Return (get-clipbrd)))
         (main)))
      (string
       (with-input-from-string (*standard-input* (delete #\Return thing))
         (main)))
      (symbol (5am:run! thing))
      (pathname
       (with-open-file (*standard-input* thing)
         (main))))))

#+swank
(defun gen-dat ()
  (uiop:with-output-file (out *dat-pathname* :if-exists :supersede)
    (format out "")))

#+swank
(defun bench (&optional (out (make-broadcast-stream)))
  (time (run *dat-pathname* out)))

;; To run: (5am:run! :sample)
#+swank
(it.bese.fiveam:test :sample
  (it.bese.fiveam:is
   (common-lisp-user::io-equal "3 2
"
    "9
"))
  (it.bese.fiveam:is
   (common-lisp-user::io-equal "3 200
"
    "10813692
"))
  (it.bese.fiveam:is
   (common-lisp-user::io-equal "100000 100000
"
    "742202979
")))